I was in Dominica in April of last year, and water was never really below 78 degrees or so, even at depth. Visibility was 35 or so at only one dive, usually 50 foot plus. But the diving, especially off L'Abym, is deep, with a steep wall slope that provides a lot of shadow, so I do think you'll want a strobe. You can check out my flickr link for my Dominica gallery - I was shooting with two DS50s, and wished I had more light quite frankly. The background colors are really vibrant with the coral over all the dark volcanic substrate (Dominica's reefs are geologically quite "young".)
I was comfortable in a 3/2 shorty, as was my wife. The DMs were really lean, and they wore I believe 3 mm fulls and did report being a little chilled after the 2nd dive.
L'Abym is actually the name of one of the dive sites, the actual marine reserve is named differently (Soufriere I think?). My favorite sites, if you get a chance to choose, were L'Abym wall, Danglebens Pinnacles, and Swiss Cheese.
Enjoy the trip! Who are you diving with? One of the dive shops, last I heard, was being run by a wetpixel member and a formative influence on getting the marine reserve set up, handle here is 'nudibranch'. And I believe there's a DM/operator from St Vincent on the board too, name escapes me, but do a search and I'm sure you'll find her.
