Adding to the previous comments, I have been using the latest EX series PL filter from Kenko, the light loss is now around one stop.
You should be aware that the AWB of some DSLRs don't deal that well with PL filters, it may become useful to shoot a reference grey in the scene.
Speaking about filters, you may also want to consider a variable neutral density filter, they help with water movement and to remove crowds from little densely populated scenes.
Cheers,
Bernard