Lots of good info posted above, and tons more on SB if you do a search or just scroll through the Cayman area of the board. Only thing I would add regarding vegan restaurants, Island Naturals is another very good place to eat vegan. Vivo is "upscale gourmet" vegan and a little pricey, but the food is fantastic. And if you do decide to go to the dark side, they usually have lionfish as well (which I guess is now considered a vegetable). Bread and Chocolate (also mentioned above) is also a favorite of ours, especially for breakfast. It's more casual and less expensive, but it's located in town, so maybe less convenient depending where you stay. Another of our favorites is Island Naturals, located on West Bay Road about mid 7 Mile Beach - and it's the next mall over from The Strand where Fosters Supermarket is, so if you are cooking at home you can shop for food as well. Island Naturals is also less expensive and pretty casual.
And regarding water - it's almost always warm, calm and clear here, especially spring/summer. According to my dive log over the past couple of years the March water temps have ranged from 79-81. The water pretty much stays between high 70s and mid 80s year round, so I guess that qualifies as "warm" - and its usually calm and almost always clear. Today, even after a few days of heavy rain, some good wind, and a bit of chop the vis was still about 70 feet. And the water temp at depth was 81, so it might be even higher by March.
