Re: 1981 15' SeaSquirt Project
The advice from Ez is what I feared, but until someone else said it, I don't know enough about them to be sure. I would start loooking for a different boat that is trailer-less or has a junk trailer. This actually looks like it's a fairly decent galv trailer. If you can keep it until you find a different rig & determine if it has a good or bad trailer.
Spending time with your son dismantling this boat will go a long way in teaching you how they are built & how to inspect the usual places for damage. That<<---- is worth a great deal.
Does your family like to container garden? I see the biggest raised flower/veggie garden in the neighborhood!
Pull the deck (100%, no 2"-4" lip) & inner transom skin & remove all the foam & wood mulch. Get all the sharp edges of 'glass ground down smooth so there is NO RISK of injury, drag it off the trailer and let your son & his friends go CRAZY w/ any of a: shark/pirate/Robinson Crusoe theme. W/ some painted 1/2" ply for a deck, and a transom. Supported as best you can or if it'll hold up w/ out the deck, leave it out. Transom skin should have a wood brace at least along the top edge. It will be easy to break &/or sharp. 'ONLY' until you get a different rig, then it gets sent to the great big deadend road for old glass boats. Bonus: it will already be partially dismantled, and some city garbage companies will take it in reasonable sized pcs on junk day.
If you have never done any glass work, this might offer you the chance to 'learn' on a boat that won't be dependent on the repairs in a structural way(esp the transom). To answer your question in your solution #1 above, the new deck would also be encased in poly & glass pre-install, and you would grind away along the bottom of the gunnel wall up 8"-12" so that once you laid the new deck, you could 'tab' it in w/ ever larger pcs of 'glass & poly. This would go along w/ the dismantle & learn about boat construction.
WoodOnGlass uses hull & deck 'glassing detail drawings, as referenced in his signature links, I thought I had them but I guess it's on my home computer.
Continued good luck, I may be absent for a few days, and look forward to seeing how this resolves for you. All is not lost if you learn something, spend time with your son, and now own a decent trailer.
JC