BTO fall beach wedding

I’m going to a wedding in September. It’s on the beach in Maine, so I’m not sure what the weather will be like but I’m not expecting it to be super warm (correct me if I’m wrong please…😅). The dress code is BTO, however the groom (my boyfriend’s friend) specified to me that they would encourage everyone to wear black tie, and requested floor length dresses. If anyone has examples of what these would look like (style, cut and colour) I’d greatly appreciate it. Also, typically I would wear heels, but I’m not sure given that it’s on a sand beach? Thanks!!