Hi tselios,
Both the resort & the diving are highly recommended. During season, your chances of whale sharks is relatively high.
Last time I was in Placencia I was diving in Gladden Spit an outing specifically aimed at finding whale sharks. We hung in a tight group at 120-140' with the hope that our collective bubbles would look like Cubera snapper spawn (it was the season) and draw whale sharks out of the deep. Needless to say, this didn't always meet with success. But one dive it did and we ended up seeing a total of 4, but the premier sighting was watching a 25 footer silently easing on by with the sun glinting off its two-toned pattern of light spots on its bluish-gray back and white underside when a group of 4 dolphins, including a baby, swam over and started playing in front of its snout. Wow, it like knocked my socks off.
Robert's Grove is excellent, if pricey. Staff cannot be beat. Food can be uneven, but if you're not happy with it there are some cool & inexpensive places in town.
Have a dive circus.
Cheers,
DocVikingo
And, yes, Placencia is amazingly different (read as undeveloped & personal) compared to Ambergris Caye.
Have a dive circus.