simply play them, or edit?
If you're wanting to edit them, the watermarks are there for a reason - so you can't edit them, and so credit is given in the right places. You shouldn't be trying to edit out watermarks.
However, if you're wanting to simply play them, I'd recommend flip4mac. It allows you to play wmv files through quicktime, and even in your browser. The alternative is the mac version of windows media player, which is just plain slow and cumbersome. There's no real way to turn watermarks off, as they're part of the image.