Belize, Honduras, Cozumel are generally cheaper than most other Caribbean destinations but it looks like they are off the list due to having been there or seasonal weather.
As said then Bahamas out islands (that you haven't been to) further south might be an option. There are alot of them. I have not heard of ferry service.
You might try Mexico on the other side (La Paz, Cabo San Lucas). I haven't been to that side so it's a suggestion. It is drier there though. I did have a friend who dove it and said it wasn't bad. Wide beaches he said but colder water. Haven't heard of ferries to other sites tho.
Pacific might be an option like Maui and you could visit other islands. Could be expensive and I am not sure about interisland ferries. I have not been to the Tahitian islands but all my research suggests it's a pretty penny - I am still trying. Fiji can be fairly inexpensive (or expensive) depending on how you do it. We were in the Taveuni area. It is however, alot further away than the Caribbean.
Too bad Cozumel is off you list. There is ferry service to the mainland and you can also get to Isla Mujeres and Holbox for the whalesharks relatively cheaply.
Maybe some research into Costa Rica or Panama might yield some answers.
The rainy season part is throwing me a bit. Oct tru Feb is one of my favorite times to go to Cozumel. It is in the general region of Belize and Honduras and I think the weather patterns might be similar. Except for Nortes in Dec-Feb, the weather can be fantastic. We have been there in Oct, Nov, Dec . It can rain on and off but I have been there in Apr and May where it rained for a week straight.
I wouldn't put too much stock in weather reports based out of the US for Yucatan and Central America. It may rain heavily for a bit then stop. Passing storms and fronts can be completely different. You might search local weather archives (google search). Though most forecasting sites I have seen are generally wrong the weatherunderground site seems better but search others.
Out of curiosity I did a Roatan Weatherundergound search on Roatan Intl Airport(Roatan page then trip planner) for Dec 1 -10. It gives data from 1996 to 2009. The link is here -
History : Weather Underground
Note that the precipitation numbers are all zero. I suspect it wasn't measured or reported. However, noting the rain events, there are only about 2-3 avg rain events per 10 day period and some with none. I don't know what information you have but compare them and to others you search. I wouldn't eliminate Roatan unless you've already been through this exercise. You might be missing out on a good trip.
