Was there years ago, so not sure if changed. Cougar Creek looked to be the spot for camping, if you're fishing within the sound. Apparently fantastic fishing 5 minutes out in front in August (we were there too early in July). Depending on boat size, if you can sleep-aboard, you could see if Critter Cove has mooring space available. The cabins are quite expensive, but moorage was pretty affordable (like $30 a night if I remember right).
You can also camp at Friendly Cove, and tie up at their dock. This would be awesome if you plan to fish outside or at the opening, as you're right there. It is also a pretty cool place to be ... good hiking, very historic, and probably hanging out with interesting European hiker types rather than the cougar creek fishing crowd. ;-) They also have rustic cabins for rent. But when I looked into it a few years ago, the overall price ended up being comparable to staying at Critter Cove, so that's something to consider too.