Any ideas?
 

The top side of the deck, about a foot from the starboard wind deflector, where it curves to the flat part of the deck, seems to be an air bubble, in the fiberglass, about a foot in circumference. The bubble goes up and down when one steps on it. It seems to be solid behind the bubble. It is a 2003 35 ts. Is the deck fiberglass or some type of plastic? The thickness of the material seems to be about an 1/8 inch thick.

Re: Any ideas?
 

Sounds like delamination...The composite skin has seperated from the coring beneath. If this is the case, it's not a good condition. It could be from resin starvation, contamination which has not allowed proper adhesion, or interlaminar tensile stress. Either way, it needs to be fixed.

Re: Any ideas?
 

BajaDaze is right. It will continue to expand. Depending on how bad it is, a temporary fix may be to drill a small hole ( I am talking injection needle size)and inject some WESTSYSTEM Resin into the area and keep it compressed until it bonds.