strikesystems - i'll second what pablosandiego said. dennis on our black river/pelican bar trip mentioned that they always see some dolphins.
now, it's not crazy acrobatic sea world(R) type stuff. no shooting out of the water in a big display and flipping and all that... we saw about 8 or 10 "pods" of dolphins - groups of 3-10 each pod, just swimming along the surface, their dorsal fins rising and falling... still a nice sight tho. :)
dennis also mentioned that if the seas are rough, that drives the baitfish deeper and thus the dolphins - but if the seas are that rough, your trip would probably be cancelled anyway (if you're doing the fishing boat type trip from trasure beach/jake's area, along the coast and up the black river and then back to pelicanbar.
when we went, seas were probably 3 feet or so - nothing too sketchy. you'll have a great time. but the sun is VERY strong on the water. polarized sunglasses are recommended and a waterproof sunscreen will need to be applied a few times during the trip if you're fair-skinned.