Why does P2Pool show up on BitcoinWatch.com if it's P2P?
All miners in P2Pool mine "solo" but distributing the reward between all its peers, hence having both advantages of solo mining and pooled mining. The reward distribution is build into the block itself, you can't mine such a block to change it later to give yourself all the reward, nor will the P2Pool accept shares that don't contain reward redistribution.

If it doesn't show as a pool it's encased into "other known".