A test using a population of a simple boat model (no shadows, no GI and stuff!). It's just one instance (a bit tricky, because like Ulco said, sometimes some more instances are appearing and disappearing. They have to be deleted in edit mode.)
For the crests I used a trick. I rendered an image sequence of the crests from straight up with an additional camera, edited this sequence in AE to get some sort of trails, and used the same camera again in an image map shader to project it back on the surface as a mask.
Still not happy with it, but I'll keep on fighting!