I am going to suggest that you perhaps postpone your trip by a month and plan for the full moon in April as whale sharks come in during spring months. Might see them in March but April - May is better.
Seeing them requires an outfit that has a permit to dive at Gladden Spit Marine Reserve which is quite regulated.
As for places we dove out of Placencia and had
great accommodations. We and others found the dive operation at Seahorse to quite rigid and controlling to the point that I would not recommend them. Mid week I had long chat with Brian jr the owner and once finished with the week we left zero tip.
Post up as I have read good reviews on SB about them.