I've done downwind ocean paddles and mild white water river paddling in my Nelo 510. It's obviously not the best boat for either of these types of paddling but it's not the worst either. You would do well to take advantage of the interchangeable rudder though. The over stern rudder is pretty ordinary in any kind of following sea and you obviously won't be using the under stern rudder in white water.
I find that my Nelo 510 has a bit of a nose down stance in the water so it feels like it is pushing more water than it needs to. My other (single) skis are literally half the weight of the 510 so it was never going to feel fast but my carbon skis are nowhere near as versatile. Of course they were also more than double the price.
I haven't had any trouble with the fabric hatch cover but if I had the choice I would get a solid cover.
I had a Spirit PRS for a while. I still can't believe I put up with that thing as my only ski for over a year.
Current skis:
Epic V10L Ultra, Carbonology Sport Boost LV, Fenn Bluefin, Nelo 510, Fenn XT double
Previous skis
Spirit PRS, Fenn Swordfish, Fenn XT, Fenn Swordfish S, Think Zen, Epic V10L Club
Most with DK rudders.