Heh, I read that first line as "So I was invited to my girlfriend's boyfriend's boat this weekend" I'm like, you invited yourself and youre now afraid to be rude to yourself?
I generally agree with the above posters. You say youre going to go, you go. occationally other circumstances can change that. Really it's up to you. If you feel like declining, just do it. You only live once, don't waste it going to things you don't want to.