Honestly, I have never heard of whale watching in the Caribbean, and it doesn't surprise me that there are no guarantees.
I assume that if they are running that tour they know where the whales have been sighted that day (although whales do move pretty fast).
The best whale watching in is Alaska in the summer or the west coast of Mexico off of Baja in the winter. Inside the Sea of Cortez is a common place and a prwetty sure thing.
If you do this and have success, please let us know.
It is important to note you don't see whales from a ship very often, so special tours are a good idea no matter where you are.