It may be OK, but the algae grows like wild! The last fiberglass boat I left in the water was a few years ago, not too many years after the zebra mussels arrived, and in two months every square inch of bottom was covered with zebras and there was some goddam thing growing that was about five feet long! I had to scrape the bottom with a board, then spray it with acid and take it to the quarter car wash to get the stuff off.
Maybe you'll have better luck, but you'll surely get some nasty growth.