For your 12', a 9.9 or 15 will be fine. I have a 20 hp on my 14' Lund and I can go from Lowell to HWY 522(at high water). I also have an electric trolling motor that I use for trolling, the 20 hp is just to get to the honey holes. There are plenty of places to anchor and fish, just look for rolling fish and unfortunately a lot of boats. Being a humpy year, the Snohomish will be wall to wall with boats and bankies. Most boats use the Maple Street launch downtown Snohomish. It is a single lane launch(not very wide, my 20' Alumaweld barely makes it) and not much parking. But it's free and there are spots to picnic. At low tide you can motor downriver to Lowell and back with no problem, but you'll have trouble going upriver past the railroad trestle just above the launch. At high tide, no problem anywhere, just go slow until you know the shallow spots and stumps.

I'm hoping to put "Snake Pliskin" on both of my boats, if you see me, say hello.