Jess, I think it totally depends on who all you have coming & how you & FI feel about it. Michelle's right in that this is a once in a lifetime opportunity - you're having your wedding on the beach!! How great is that?!?
But having said that if you have ppl coming, maybe grandparents or someone unsteady on their feet or with a physical limitation, that maybe would be able to get up & dance a bit on a dance floor but not on the soft sand then maybe take that into consideration.
I don't think you can go wrong either way, just go with your gut.