We spent a few days at Clearwater two weeks ago and DD and I went on the Dolphin Encounter trip (DH didn't come as he doesn't 'Do' boats) We paid $24 for a 90 minute trip which I thought was really excellent value. We booked on spec. at the dock. All the excursions are in the same place (just behind the main carpark at Clearwater Beach) including the Seascreamer (which looked fun but doesn't last as long as the Dolphin Encounter) and also the Pirates Cruise which was expensive and looked naff, naff, naff! Other offerings included Parasailing, which I was sorely tempted to do and also Waverunner Hire.
We booked the 11am trip but you could choose from about 4 sailings everyday (I think this is the same with the Seascreamer) The trip was one of the highlights of my holiday and we saw loads of dolphins, both in the water and chasing the boat. The scenery was spectacular and we also fed seagulls with bread from the boat which DD really enjoyed (although it was a little like that scene from 'Hitchcock's The Birds'.)
The drive back from Clearwater took around 2 hours and was really easy to navigate so, if you have a car, it may be as easy and cheaper to drive yourselves down and just book the trip when you get there.