I have the Ewa Marine bag. Not the most comfortable for putting the camera in but it works quite nice.
I like the fact that the camera is floating on the waters surface when you don't hold it.
I only used it for closeup shots on the surface. The viewfinder usage seems OK to me. Anyway you mainly go for a "lucky shot" when a boarder passes by...
Here some thest pics from the first days: http://www.mseidel.ch/pictures/thumbnails.php?album=16