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.

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.

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.