I would create my watermark and then put a lens from the effects menu.
You can set the opacity from there.
I always generate a 72 dpi jpeg for a proof. I scale down the image size to
about 12" wide before I export it to the jpeg. This makes it readable so the customer doesn't have to scroll around the screen to see it and if someone
tries to make it larger and print it, than it will be unusable.
Flexi has an export function to a PDF, and this would seem to be a great
option BUT... it includes all the vectors with it. So, if someone is trying to steal your file, you may as well send them the EPS file.