I had the same problem. Happened all the time.
Buttoning the shirt worked in terms of keeping it closed, but it also slowed down the access to the weapon considerably.
I went with a heavier-weight fabric over-layer... something along the lines of a Levi's or Wrangler jeans jacket with the sleeves whacked off, a heavy Carhartt or Levi's blue denim shirt, or one of the khaki fishing vests from Cabelas. The heavier material was a bit stiffer and didn't blow away in every breeze like the Hawaiian shirts.
For other occasions I went with a Kahr PM-40 in a pocket holster in the right trou pocket of a pair of khaki slacks.
(OTOH, I don't live in Florida....)
These guys also have some potentially useful options:
http://www.duluthtrading.com/
Best with it,
Doc