I've always understood that piston wash is a better reading than plugs, which is why I'm trying to find a good inspection light right now in order to help with tuning my boat.
Carbon build-up on the piston is from the piston being hot. If the carbon build-up is in the center that's fine and indicates a good burn. It's good to see a small build-up in the middle that is kind of granular. If you see wash all the way to the edges or more importantly to the exhaust edge of the piston it's an indication that you're running too lean and hot. This is what has been explained to me. I know racers that will set up their boats so that there's build-up all the way to the edges except for the last 1/4" around the edge.
This is from what I've been told and read but I havn't done too many readings myself. I would say yours look like your running enough fuel maybe too much but it's hard to tell as it's not too defined. In the pics that SJ/XPBri posted I would be pretty happy if mine looked like the one on the left. My pistons last year looked like yours do and I always though I was running a little rich. My plugs read a little rich and I had some black at my exhaust outlet.
