I told my friend that Mudcatters would know the name of a movie she is trying to remember. Here's what she told me about it:

A group of about five people have come to a castle (in real life, it is a well-known castle, maybe in the Bay of Fundy?) where they get into some discussions about metaphysics. When the tide comes in, the castle becomes an island. When the tide goes out, which takes about twleve hours, it is connected to the mainland and teh road to it is passable once again.

Ring any bells? Hope someone can help. Thanks!