To be honest, I would invent an excuse. Maybe it's cowardly, but I can't think of any polite way to say what you're trying to say - that they want to be friends more than you do.
Depending on your situation and how often you see them I'd say something like "Gee, I know we've been talking about this and it would be great fun, but our time here is running out and we've found out that we'd really prefer to just spend the last month at home without travelling." And then invite them to something less time-consuming, like dinner or a walk with the dogs. Or let them show you pictures of their beach house, if it's their pride and joy and they would just love to show you.
If you're really pressed for an excuse you could say your dog hates water, or boats or something... ;-)
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett