halacha - Can one use a modified palapa built on a pier as his Succah?

See this article for a description and picture of a palapa (Aka "tiki hut") built on a pier.

Assume that one puts strong curtains on three sides and ties them down firmly to the side poles so that a normal wind doesn't blow them around, and that the schach is kosher. (This question addresses non-flat roofs.)

Are you allowed to build a succah on a dock like this which is immediately above the water? Is there any rule that says that the succah must be on "firm ground"? Here, waves will rock the dock around a bit.