We went to Vero in early March in 2001. It was in the high 70s to low 80s every day. There was a cool breeze while we were there, which made stepping out of the pool a little tough. The ocean temps were cool and there was a strong riptide from the remnants of a blizzard up north, so we didn't get to enjoy that as much.
Shorts, bathing suits during the day. You could wear a light long sleeve top at night (I don't think any of us did but if you tend to get cold you might like one).