Also dive with small group size helps. We dive six pac boats and missing a diver or 2 is obvious. Diving a boat with 16 or 20 people and one less diver can happen.Don't overlook the obvious - one of the BEST ways to make sure the boat doesn't forget you is to make friendly conversation with the captain and the DM on the way out, and then have a large SMB inflated if you surface away from the boat. Knowing a few words in spanish doesn't hurt either. It's hard to forget someone who makes a good impression. Bonus points for starting a juicy story just before the dive and leaving them hanging until you get back.
We carry a SMB, mirror and flashlight. On really remote or sites in North Cozumel I carry my Nautilus Radio ver 1 that has a full marine radio. They don't make them anymore and I just had it serviced.