How's this for non-thinking, dumb moves: in an earlier reply, I mentioned 'shimming up the left side', i.e. the side with a LOWER water height. Doh! Wrong side. I should have been lowering that side! Grrh. Still, I have emptied all but about 2 inches of water. I'm definitely wanting to get this levelled out now, as opposed to six months from now. So, I'll empty the rest of the water and basically start over.
Those wooden shims definitely compress. I have some of that brown peg board -- the kind with no holes, though. It comes in a couple of thicknesses. It doesn't seem compressible. I wonder if it'd work. Matt -- where does one buy metal shims? Thanks, and to everyone who replied so far. ..Karl