This is a hard one to call. We are after all talking a bee swarm, the dolphin has an apparent advantage in its size and being in the water, and that fat layer under its skin. It will need to come up out of the water to attack, after all it is a kill or be killed thing. So at some point it will need to attack. It can try to use it's blowhole, but I think that would be rather ineffective, so the only choice is to tail walk on the water like Flipper, and then try to smash the bees into the water, but this is a swarm, eyes and blowhole are vulnerable spots for the dolphin, after the swarm takes those out blinded and enraged the dolphin has to resort to echo location to find the bee swarm, and if he is not paying attention, the bees can entice him to shallow water and possible beaching then finish him off.

So after all that, I still think the dolphin will win, because bees do not have the stamina to remain in flight for extended periods of time.