Is this a Hobie 16? If so, putting glass on the inside will not necessarily solve the problem. The foam inside the inner and outer layers of glass has deteriorated. There's volumes of information about this kind of repair...but in a nutshell; drill several holes just through the top skin, with an "L" shaped wire, spin it inside the holes to break up the foam, inject resin (epoxy preferably since you may be dealing with some moisture) with a cheap syringe and until epoxy flows out of other holes, tape over holes ,and let it cure. This usually takes care of the soft spots for a while.