Water tight filler from interlux is a good epoxy based filler that you can commonly buy anywhere. It sands easily and is super smooth and goes on like a glaze puddie. The 3m vinyl ester stuff mentioned works well too for larger spots but not as thin and smooth as the water tight interlux. There is also a ton of other products out there as well just like the one captain chuck mentioned that would work well too. Just stick with a vinyl-ester based or epoxy based product that dose not have talc in it. Talc it a common additive in fillers but it will absorb water and cause problems. Stay away from products with talc in it.
http://www.westmarine.com/buy/interl...8?recordNum=22