Marine tex is a good option as Sportster has said. so the crack does not return dont skim over existing repair, v-knotch the crack alittle to create a v shaped trough so your fill material (marine-tex,matched gel coat) stays put q-tips and tooth picks work great to add your fill material to your small repairs, sand then buff. I have had good luck contacting boat companys in the past to get a gelcoat match, I even contacted wellcraft and got a gelcoat match from 1974!!! so if your feeling froggy and want to do it right then call seadoo and get factory gel coat code then have it mixed to that code. good luck with your repair. any pics so we can see how bad? just want to make sure Im giving the right advice as far as damage goes.