OK, I found the problem:

According to

http://lists.openstreetmap.org/pipermail/talk/2010-April/049629.html

leisure=marina should not be rendered as a blue area anymore. It should be rendered as a line instead.

In the default style it is still rendered as a polygon area. And in case of the harbor of Biograd, Croatia this polygon covers some land.

I think this should be changed in the default style.

But even when I precompile sea from recent shape files or use the latest from Wanmil, I have some flooding in the harbor.