CHART-PLOTTER
It used to be that the skipper would navigate using a paper chart, taking positions from the GPS and plotting them with ruler and pencil.
An added complication would be correcting between the co-ordinate system used by the GPS and that used by the chart.
While paper charts are still carried on a charter-boat, it is rare that they are actually used, because all the charts are shown digitally on a plotter display. The chart-plotter electronics will either include a GPS receiver or get the position by cable from a separate GPS to show the boat's position and course on the plotter.
The skipper can save the co-ordinates of thousands of wrecks into the plotter. Selecting one as a destination will then display the course the boat needs to steer and the expected time of arrival.
Most GPS receivers can also be set up to do this, but it is so much easier to do it on the chart-plotter, especially when it can superimpose all sorts of additional information.

ECHO-SOUNDER
An echo-sounder gives a representation of the seabed beneath the boat. In its simplest form, it sends out pulses of sound below the boat and measures the time it takes
for an echo to return. Knowing the speed of sound in water, this time can then be equated to depth.
Modern echo-sounders are far more complex. A single pulse from the echo-sounder can result in multiple echoes. Depending on the object from which they are echoed,
these can be short and crisp, or soft and spread.
Slight variations in sound frequency can also change characteristics of the echoes returned. All this information is processed and displayed as colours or levels of grey that enable the skipper to see the wreck below.
Many echo-sounders have a dual frequency capability.
A high frequency gives a narrow beam below the boat for precision. A low frequency gives a much wider beam, so is often used on the initial search for a wreck, before the skipper switches to high frequency to drop the shot.

AUTOMATIC IDENTIFICATION SYSTEM
The Automatic Identification System (AIS) is a device that's kept out of sight. It comes in two varieties.
Any ship of 300 tons or more is required to carry a transponder, which transmits a VHF signal containing its identity, size, position, course and speed.
AIS is not required on smaller vessels, but many charter-boats carry an AIS receiver, either integrated into a chart-plotter or marine VHF radio, or as a self-contained unit plugged into the chart-plotter.
Quite a few carry full transponders to transmit their own AIS data.
As well as having other vessels with all their identity and course data displayed on the chart-plotter, AIS will automatically alert the skipper of possible collisions.
With divers about to jump in, the AIS can be used to check whether any vessel in range would cross the dive site. Reading an approaching vessel's name from the AIS data, the skipper knows who to call on the VHF radio.
There are even websites that plot vessels on a map from their AIS data.
If you know that your favourite dive-boat has an AIS transponder, you can see where it's diving even if you're stuck at home. Try visiting www.marinetraffic. com to see ships with AIS superimposed on a Google map.

COMPASS
The most obvious compass on a charter-boat is the floating ball located in front of the helm where, in a worst case with no electronics, the skipper can use it to steer the boat by hand and eye.
However, the compass that is actually used most of the time will be another electronic device, typically hidden behind a bulkhead or below the wheelhouse floor, sending information on the current heading to the chart-plotter and auto helm.
While this compass is an electronic instrument, it still works by measuring the Earth's magnetic field, and can be misled by nearby metal and magnets, like the steel caps in safety boots.
On some boats, I have managed to steer the boat in circles by dancing on the wheelhouse floor, directly above where the compass is hidden.

GPS
The Global Positioning System was developed for the US military, then made available as a public service.
A GPS receiver calculates a boat's position from a network of satellites orbiting the Earth. The satellites constantly broadcast radio signals containing their orbital location and the precise time. A receiver measures the time delay between transmission and reception to calculate the distance from a satellite, then uses distance from several satellites to locate the boat.
Ideally, the distance from three satellites could give a 3D position. In practice, the clock in a GPS receiver is considerably less accurate than the clocks in the satellites, so three is too few satellites when considered against the very short time delays actually measured.
By including time in the calculations as a fourth dimension, signals from four satellites are used to give an accurate position. GPS receivers will typically have 12 channels, so use up to 12 satellites at a time to increase accuracy to within a few metres.

AUTO HELM
The auto helm is a compact box of electronics that connects to the steering of the boat, hidden behind the helm or steering-wheel. When engaged, it will turn the rudder to keep the boat heading on whichever course the auto helm has been set to steer.
To do this, it needs a reference of the actual heading from a compass. When also connected to a GPS or chart-plotter, the desired heading can be constantly updated so that the auto helm corrects for any leeway where current, wind and waves would otherwise have pushed a boat sideways and off course.

RADAR
Radar sweeps a circle about the boat with a radio beam, picking up reflections of this beam from other boats, large rocks, navigation buoys, rain-showers and anything else big enough to give a decent reflection.
Radar used to be displayed on bulky circular displays with a rotating marker and blips that slowly faded until the marker next came round. Modern displays are flat-screen LCDs, like everything else.
An opportunity for confusion is that the top of a radar display is traditionally the heading of the boat, not north as on a chart-plotter display. However, with the radar system taking an input from the compass and GPS, this can be adjusted to align north
at the top of the display, or even to superimpose the radar information on top of the chart-plotter display.

MARINE VHF RADIO
This enables the skipper to talk to other boats and the Coastguard. When transmitting, the signal is broadcast, so anyone tuned to the channel can listen in. Multiple radios transmitting on the same channel will interfere with each other, and prevent intelligible reception.
To prevent this, radio communication is managed by adhering to correct procedure, which involves using the appropriate channel and words such as "over" to indicate that a transmission has ended, and it is the other party's turn to talk.
Channel 16 is the official channel for making contact, and also for the Coastguard to manage emergencies. Once contact has been made on Channel 16, both parties then switch to another channel.
In most areas there are general agreements that all charter-boats will work on a particular channel, while commercial trawlers and yachts use other channels, so keeping traffic away from 16.
The marine VHF radios fitted to charter-boats include Digital Selective Calling, where a digital signal can be sent to alert another vessel that you want to speak to it. It's a bit like making a phone call, except that once connection is established, communication proceeds on the usual open VHF radio frequencies.
The "phone number" is a boat's Maritime Mobile Service Identity (MMSI), available
from AIS data.
DSC radios also have inputs for position information from a GPS or chart-plotter. In an emergency, the guarded red distress button can be pushed. The effect is to transmit a distress signal with the boat's position.

NOTEBOOK PC
Rather than using a specialised chart-plotter or multi-function displays, some skippers prefer simply to run the appropriate software on a PC, with inputs from the other instruments. The PC can then show all the information that a specialised chart-plotter display or multi-function display would reveal.

THE GLASS COCKPIT
You may have noticed that all the latest electronic systems use colour flat-panel LCD displays. This has not been lost on the designers of marine electronic systems. The ultimate in charter-boat electronics is a glass cockpit, as they have on commercial aircraft.
In front of the skipper will be two or three large flat-panel multi-function displays. These can be configured to show anything from the various electronic instruments described, to other stuff such as the engine instruments.
A skipper could have a chart-plotter with radar and AIS on one display, and split the other between windows for the engine instruments and the echo-sounder.
Close to the wreck, he will resize and swap windows to give a bigger echo-sounder display while the wreck is located and the shot dropped.