smallboat

You have a blistering problem.  This occurs when a boat is kept in the water as opposed to on a trailer and the fiberglass lamination becomes wet and separates from the gel coat.  I suspect the previous owner kept the boat in a slip. Yes, fiberglass hulls made using polyester resins can and will absorb moisture.  To repair, grind off the gel coat surrounding all of the blisters.  Allow the hull to dry out - use a moisture meter to verify.  Fill the blisters and areas you have sanded with a filled epoxy mix.  Since this appears to be a cosmetic repair, use glass beads or micro balloons as the filler. After curing and sanding, paint the entire bottom with a barrier coat paint - Interlux and others make them.  This is an important step.  Follow with a good topcoat paint.  You don't need a bottom paint unless you plan on keeping the boat in the water.  The new two part linear urethanes are extremely tough and will work very well.  Blisters are mostly a cosmetic defect but can lead to greater problems if the hull lamination becomes saturated.  Not typically a problem with trailered boats but fix the problem now.

Water absorbed into the hull lamination will not effect your stringers and foam unless you have a hole in the boat.

Just got in from Niagara red buoy. 3 to 4 footers all day with the tops now blowing blowing off.  Wind picking up from the NE.