first off, polyester is a casting resin so it's fine for making a casting such as a boat hull
epoxy is a adhesive that also acts as a casting resin but will actually stick on a molecular level to whatever you may be sticking to

to fix your crack: find a old fashioned juice can type can opener
the one shaped like a triangle point or something similar
gouge out the crack so that there is a v shaped valley down the length
then drill a 1/8 -1/4 hole at both ends of the valley that you gouged out to stop the crack from elongating
put blue painters tape on the outside of the hull to keep epoxy from running out
mix up your epoxy and give it all a coat then stir some high density filler into the remaining epoxy and fill the valley up
let dry and voila
west systems brand epoxy is availabe pretty much at most marina's and is a good brand and the filler is from them also
if you can't find it locally the internet has plenty

using polyester to fix polyester is not the correct way to make it last