First of all, thanks for taking the time to reply to my thread. I appreciate all of your suggestions/thoughts.
I've owned one of these boats and these compartments are filled with foam through holes in the top.
But there are holes in the bottom for drainage, they are not sealed. The only way you could fill these to the top would be to slowly freeze them and then leave it exposed to the winter rain or snow.
But I can't imagine how long this would take.
Was the boat submerged at one point?
Ricohman is correct here. These are factory enclosures that are filled with foam (likely through the holes as ricohman suggests) and the 'boxes' are riveted to the hull and floor, so they can't be opened without drilling the rivets.
To address the speculation about how long it would take for large foam blocks to completely absorb water, I can venture a guess: 5-7 years of being uncovered, with the holes unplugged, and the stern of the boat filled with water and debris over the winter. Aside from drilling the rivets and removing the foam blocks like Watermann correctly suggests, I guess 5-7 years to let the water seep out the bottom with the holes plugged is the easier, less effective approach :chargrined:
Fortunately, the boat, engine, trolling motor and trailer were all given to my son (knowing that I'd get it back to snuff). The neglected Johnson 30 now runs like new, and the boat looks great, thanks to countless hours of power washing with bleach detergent. The waterlogged flotation boxes are the only issue. An issue that we can live with, I guess.
..and the boat will now be properly covered for the rest of its life.