One of my favorite underwater things to do is to find a fish that is camouflaging and then see if I can look away and then back and find it again :) Lizard fish is one of the great subjects for this game.
Juvenile Spotted Drum fish was very hard to draw - it kept turning and even though I spent a good while trying to capture it the result is very poor.
Oh - and another interesting thing to do underwater is if you see a fish that usually stays with it's partner to find that partner and observe them together. Like these two Rock Beauty Angels (yellow and dark blue-to-black) snacking on a large porous vase sponge and chasing each other at the same time.
Original underwater sketches are taped in my regular sketchbook and comments and drawings around are done on the dry land a bit later.