Hi,
Being one of the co authors of the Cayman Nudibranch Book, I can offer up some hints which hopefully will give you some what better odds than a just crapshoot.:) Cor and Tursiops hit the main ones.
Forget the guided boat dives! You need to move very slow stopping frequently to really look. Ideally shore diving works the best. Most of what you see photographed in the book was found in the 15-60ft depth range, with the highest concentration being in the shallower depth. Boat dives are going to be too deep for you to spend adequate time looking. Hopefully you have a like minded buddy.
If I'm looking for them during the day, then I'm focusing on grasses, rubble, debris,( Tritoniopsis frydis love to congregate on dead sea rods) hydroids, and algae. Its pretty amazing what all is hiding a top the hard pan at Turtle Reef that divers never see on their rush to get to the mini or main wall. Look for egg coils on algae clumps( light fanning above the algae reveals more) usually a tell tale sign nudis are lurking .Don't forget to think small, and smaller. Those Nudibranchs in the Caribbean don't begin to compare in size to their Indo Pacific cousins, but it makes finding the little buggars even more special.
At night, its all about the sponges, all varieties. That is where you will find the greatest concentration of nudibranchs out feeding. Each variety has their preferred dinner of choice. Trapania dalva seem to like Brown Bowl Sponges, Tritonia Bayeri Sea rods, etc, They'll be on the algae as well, and at night as opposed to being buried deep within a clump they are most likely riding on the top edge of a leaf. If I was focusing a trip around finding them, I would plan on doing as many night dives as I could, multiples in one evening. Start just after dark, do one, get your SIT in and do another after the crowds have gone home.:)
Be patient. They are there. If you know where and how to look. Familiarize yourself with their dietary habits and look to their food source.
If you decide on Cayman, and have other questions feel free to message me.
Happy Hunting!
Cindy