To get a story-like structure, we needed a character around which to thread the programme. The humpback whale typifies the contrasts in the shallow seas, from tropical to temperate.
Humpbacks come into shallow seas in the Equatorial belt to give birth to their calves - theories about this include the fact that there are fewer predators, calmer waters and warm-bath temperatures, making it a good place to grow up.
We had seen footage from Hawaii but wanted to push the envelope by going where there were fewer tourists and fewer restrictions.
We went first to Rurutu in French Polynesia, hoping to dive with a mother and calf. It was far from a tropical paradise, there was a lot of rain and we were all shivering.
We spent a month going round this small island but none of the whales had calves, although we did film a male singing, which opens the show.
We wanted to get beneath the whales to show their grandeur but without disturbing them in any way, so we used Draeger Dolphin rebreathers and took the equipment to compress the gases ourselves.
The following year Doug Allen went to Tonga, where the whales are more out in the blue, and filmed calves while using a snorkel for mobility.
He got some really intimate, cute footage of babies frolicking, taking their first breaths - lovely behaviour. We also used footage from Hervey Bay in Australia and from the Aleutian Islands (Bering Sea) at the end of the film.
There are still massive gatherings of humpback whales in the Aleutians, as there once were at Grand Banks, and though its so rough up there we captured the magnificent spectacle of the whales gorging on krill and herring.
The only real incident occurred when Sue Flood was swiped by a calf. She was taking stills and it came to her, very playful and curious, and bumped her as it turned. A bump from a 3m-long calf can be quite significant!
We ended up with a story illustrating how the mother and calf are forced out after six months because the mother is starving but the baby is now strong enough to make the journey. It includes fantastic aerial shots in rough seas.
We wanted to move on from Blue Planet, and were able to film very strong scenes in Raja Ampat (West Papua, Indonesia). This is one of the newly discovered centres of biodiversity - the richest reefs in the world.
It was far from idyllic filming there. The currents are too strong for that, but this is why the species are so rich. Peter Scoones used a special underwater housing for a high-definition camera to film the strangest creatures and show how well-adapted they were.
One sequence shows pygmy seahorses head-butting one another, tiny males settling some territorial dispute. Then there was the electric clam, which uses these little hairs to reflect light - one theory is that this is to deter fish from nibbling it.
But the amazing sea snakes in the Banda Sea provided one of the most exciting stories, and was a real coup for us. We found a 30-strong band on the hunt with jacks. The sea snakes were trailing the jacks as they drove little fish into crannies in the coral heads. The snakes would go in and flush them out for the jacks to gobble up. This is the sequence of which Im proudest.
Only 6% of Indonesias coral reefs remain pristine, so there is a big incentive to film things while they remain. The rapid decimation of the ocean is quite depressing.
We managed to capture new behaviour using a new piece of time-lapse technology, a digital reflex underwater stills camera with inbuilt timer mounted on a tripod.
When you set it to take a still every 10 seconds and put the results together, what appeared to be lifeless comes alive. We used it to film starfish and sea urchins, and to show coral polyps emerging at night.
We wanted to tell the story of how urchin plagues affected the great kelp forests, and Peter Kragh and Bob Cranston filmed in this way at places such as Star Gardens in North California and in Raja Ampat.
The sunflower stars in particular look very comical, like something from Morph or The Clangers, chasing brittlestars that part before them like the Red Sea.
The giant sunflowers, which can grow up to a metre wide, eventually munch their prey on a bed of sand dollars.
Doing this sort of filming was a learning curve over the period of three years it took to make Planet Earth, but now people with semi-pro SLR cameras may want to start making their own films of this sort. Technically, I think this sequence in the programme was the most satisfying.
We wanted to get aerial shots of dolphins surfing into shore at Byron Bay [in eastern Australia]. People told us that the dolphins did it for fun but when
I went up in a helicopter with a scientist we found they were using the technique to get at baitfish in the shallows.
They would do backflips to stun the fish and grab them. As we saw later when filming in western Australia, they would have to get back out in the water before they were stranded by using their pelvic fins. Eight known species are thought to have developed this surfing technique, hydroplaning in just 15-20cm of water.
We tend to think of the tropics as these rich seas, but apart from oases in the form of seagrass beds and coral reefs there is not that much life - which is why the waters are so clear, and why these dolphins have come up with such a risky technique to feed. Soupy-green temperate seas are so much richer.
For six weeks of the year, great white sharks patrol Seal Island to ambush the seal pups. Every morning the pups leave the feeding grounds hell for leather, and we were able to trail them with our ultra-high-speed HD camera.
This records everything onto the hard drive up to the moment when you hit the button, so if a great white leaps out of the water and you hit the button, you know youve got the shot.
We spent a month out there and finished filming only this August. This is why Planet Earth is split into two halves - there was no way we could have finished everything in the time, and our hope is that by splitting it, peoples appetite will still be there.
Shallow Seas is one of six new Planet Earth programmes airing on BBC 1 at 9pm on Sunday nights. The series, which also includes Open Oceans as well as Polar, Plains, Jungles and Forests, starts on 5 November, with Shallow Seas going out on 26 November.