I don't know about weather or why rates are low other than a lot of people would have just taken advantage of the opportunity to travel over children's spring breaks just weeks earlier?
I will say that we are considering going the first week in April and that between March and early June there can be sea lice (thimble jellyfish larvae that can't be seen, super tiny) in the water near the turtle grass in CC. Read about them before our cruise in June several years ago and bought a sunscreen called Sea Safe which helps protect against sea lice stings. Son didn't use much of it and did get a few stings and had irritation in the area. Nothing major and wouldn't keep us from going again. Just read up on them( I think on this board there is even info.) and protect yourself, then jump in the water and have fun!