This sounds like a job for the Q. It doesn't weigh much so it won't throw your balance off, and if you throw the adapter on, your 18-135 becomes something like a 100-750 or 85-650 equivalent (depending on which Q model you use), or you could use a cheaper manual zoom or normal prime lens for telephoto. Or you could get the tiny but potent Q 06 lens, which is about 70-200/80-250 equivalent and will autofocus. (Might need a screen loupe though since its not the best screen in bright light. )
That way, if you do drop it, you're only out maybe $100 for a used Q body (and however much a 06 lens goes for if you're using that). If you can recover it, since an adapted lens wouldn't be using any electronics, it probably won't have damage from that if you clean and dry it out correctly.
I'm trying to decide if I want to risk taking my K5 on a canoe trip. I can see why (besides image quality) normhead is such a fan of the 18-135. Don't want to change lenses when they might be wet; the 16-50 isn't long enough, and the 50-135 isn't wide enough. I'll probably take the Q the first time and if it goes well, bring the K5 the next.