Filling the hull with foam

Very new to the laser world. I was gifted an old model laser with a few issues and need some help.
The first issue I had with the boat was the deck was separating from the hull. I with some PL Premium and some pop rivets, I hastily put it together and put the boat in the water. After being tied to the dock For the weekend, I pulled it out and there was 5 inches of water in hull when looking through the stern inspection port. Not having the ability to repair it properly, can I fill the hull...

Filling the hull with foam