There are plenty of nice dives in the Hol Chan[sp?]marine park even if it's rough. They are a short ride.
I would caution you against trying the Great Blue Hole which is 55 miles out if weather seems bad.
We did it under those conditions last year in what was almost a monsoon. Visibility was poor and sea conditions were ~ 12 feet which caused many aboard to turn to religion. It is a gotta-do-once tho under better weather.