Well the viz is going to ruin you!
The diveboat will go to the site suitable to the worst diver onboard. So if you are with a newbie you’ll get a shallow site the newbie crashing into can’t hurt :( If there are a lot of divers they might divide them up by experience.
Definitely bring the sausage, they require them and you know yours.
You’ll need the light if you’re doing a night dive but you won’t see more seahorses than in Shark River anywhere. On a day dive the light might come in handy looking in holes but I’ve only seen one swim thru in Cayman but there are wrecks. If you’re onboard with newbies they probably won’t take you inside wrecks.
Boots could feel better going up the dive ladder.
I did take a knife on a cruise in Greece, got a free ride in a police car to the ship from some guys with no sense of humor :(
3mil is what I always take and I only got frozen once by an upwelling :(
Have a good trip!