In the last year we have been to Clearwater Beach, Daytona Beach and Myrtle Beach.
Clearwater: beautiful Caribbean like water.... very clear, very calm. Water reminded more or Aruba. It was lovely. Great for young kids. Its more pricey, IMO than Myrtle.
Daytona: We were there over April Break. My Mom lives there. We stayed with her. We enjoyed Daytona Beach Shores and Ponce Inlet. Less crowded, more family like. It was fun to drive on the beach. However, that being said, it can be scary to have kids walk on a beach with cars driving on it. While the water was "rougher" than the Gulf, we found the water to be pretty clear. (keep in mind I live in CT, and the shore is all long island sound, which is really dirty, IMO). We loved it. Kids loved it.
Myrtle Beach: We went in late May. the kids LOVED it. We stayed in a condo that had multilpe lazy rivers, pools, hot tubs, kids pools and was ocean front. There was so much to do, even with the unseasonably cool weather, we were busy enough right at the hotel! Broadway @ the Beach was fun! The water, hmm... its the Atlantic, so it had lots of waves, I wouldnt call it dirty, but when we where there, it was sandier than Daytona (but there was a huge storm system off the coast from NC to FL), causing higher waves, etc.
We stayed at the Compass Cove. We loved it!
Of all places..... the only one my kids are BEGGING to go back is Myrtle Beach. It ranks right up there with Disney to them, which IMO is saying a lot! They loved having so many pools, etc in addition to the beach. And Myrtle Beach is pretty cost friendly. In fact, we are going back again in August (and trying a different resort- Holiday Sands @ South Beach condos (got a great deal and we checked it out when we where there in May).
Enjoy where ever you go!