Re: Any good techniques for cleaning the bottom of a boat that sits in a river?
If you don't want to get wet and are really obsessed you could block up your boat and get the trailer out from under it. There are many threads scattered about on just how to do that. I'd be leery, though, of undertaking that much risk for the sake of cleaning a couple of stripes a foot wide.
You could also load your boat purposely a bit off centre on the trailer so those areas are exposed, then clean, then reload properly.