Filling a gap on fiberglass 420 sailboat deck

Oh man, is the boat in an unlit garage Those pictures...

You can patch those cracks. A Dremel is probably too small. You will want clean fiberglass on each side of the crack to make for a strong patch. A power sander or maybe a flap disk on an angle grinder would take care of things fast.

 

That air channel is probably there to stiffen the cockpit. You think the whole hull was full of water and then it froze? That's a lot of water. Or maybe some water pooled in the cockpit and froze (makes more sense.) Did you just buy this boat?

 

I was thinking water trapped in the air channel because the delamination was the worst on the border of it. Would 4 or 5 layers of 6oz fiberglass cloth be structurally sufficient? I used a Dremel to open up these cracks.

 

Also, for the size of the gaps would it make sense to buy a gallon of epoxy?

 

Epoxy stores well and you'll likely need more later. 6 ounce is fine, though you'll need more layers to bulk it up. Use the mini disk grinder on the Dremel and make a shallow concave path along each crack, maybe a 1/2" wide on either side of them. Grind down through the gelcoat and feather it out as best as you can.