There is lots of Class III in way north Cali. My wife and I have done several spring break trips up there. We are Class VI boaters but stuck with more III-IV so if it was just the two of us we would be able to boat scout most everything. Wouldn't have to bank scout too much. The area is remote (I'm talking NORTH cali, San Fran is not north cali) so you can't count on hooking up with boaters on a weekday especially for class III. There may be people doing the 4-5 stuff, Nordhammer CG on the Cal Salmon is good for boaters.
The main Klamath below Yreka is II-III with 2 IVs and a VI for a hundred (?) miles. The Scott has a III section above the 4-5 part. Indian Creek from below Puppy Falls to Happy Camp is III. That is the first run we did there and after we left bikes in town and drove up and climbed down to the creek we saw lots of big steelhead and/or salmon in the big deep clear pools below the falls waiting for higher water. Just knew it wouldn't suck too bad.
Lower Clear Creek is III. S. Fk Salmon below Methodist Creek is III. N. Salmon and the main below Butler Ledge is IV. Trinity has some III runs higher up if you are going to jog around to the coast to ocean surf and head north to the Smith. South and Middle Smith (with a IV) have III runs, the North Fork is IV (V at high water).
Lets see, then head north along the coast to the Chetco near Brookings, OR and then the Elk, Coquille, Coos...
We have three weeks, how about you? Camping is great unless it is rainy which means there is water, maybe lots.