SH,
Best bet is at Skomakowa. On the west end of town immediatly after crossing Skamokowa creek there is a county park called Vista park. Right on the river, nice facilities, restrooms, and a launch on the creek. The launch is fine unless you have a deeper draft boat...then you'll have to wait for half tide to put in and out.

It is technically illegal to camp on Tenisilee. Most people seem to put up shelters for day use, but signs and regs state that overnight camping is prohibited. Not sure if it is actually enforced but thats besides the point.

There is also Countyline park between Longview and Cathlamet in addition to the Marina.