I can only speak for early February, when we sailed out of San Pedro, as the conditions were PERFECT. But others' experience just a few weeks earlier were not so ideal, as a cold front had passed through and made for unsettled conditions. I suspect that spring would be great.
We sailed with TMM, and I had many chats with Brian in their main office. If you are planning to sail with them, I recommend that you give them a call (or drop them a line) -- they are a font of excellent information. (I suspect Moorings would be the same -- I've sailed with them before but not in Belize).