Crawfish, also known as crayfish and spiny lobsters, are the country's largest crustacean but are diminishing in number due to overfishing.
Marine life research organisation Seasearch, affiliated to the Marine Conservation Society, wants to build up a better picture of what is going on.
It needs more information about populations in shallower waters, so that it can "identify hotspots where protection is needed".
Divers are asked to report where and when they see crawfish, noting size and, if more than one, numbers.
Seasearch asks for any crawfish found to be left exactly where they are, as they are now a "relatively rare species".
Reports can be made online. Go to www.seasearch.org.uk and, on the home page, click on Seasearch Recording.