I'd use the Marine-Tex. The second won't fully harden up. Either way, you've got to use an epoxy product so it will adhere to the inside surfaces. If you are talking about a soft deck, I wouldn't worry about the foam. I just did my boat and the foam was very crumbly disintegrated anyway.

I'd recommend buying more epoxy then you think you'll need, then return what you don't open if possible. I don't remember exactly, but I'm sure the 4oz kit won't be enough.