Zip-lock bags are your friend. Use one to hold several pre-oiled patches, and another to hold dry patches. Stick a cleaning rod in as well, so you do not end up leaving dirt, bark, or sap from the twig you used as a field expedient. Field strip, dry, and oil your handgun every night. Check it over first thing in the morning for condensation (which you will get as much of when you keep it in your sleeping bag with you as when you leave it outside your sleeping bag). Dry and oil as necessary. Yes, doing it both at night and in the morning is necessary if yu want to keep Mr. Rust away.
Double zip-lock bag your extra ammo. Maybe a smaller zip-lock bag for the ammo and then a bigger one for that to go in - do not push all the air out of either bag, you are hoping for the ammo to float if it does go overboard.
Invest in a lanyard - even if it is home-made from paracord or shoelaces. It is better to pull on your end of the line and bring the wayward item back aboard than to try diving for it or moaning and mourning your preventable loss. Just do not tie your lanyard to the trigger guard. (Yes, I have seem people do that. It causes problems.) If your handgun goes for a swim, dry it off and oil it as soon as possible. If it just gets rained on you can probably safely wait till evening - the folks in the .mil do.
IMHO the Glock with the Serpa holster is a better choice as you will not be getting leather wet. However, if you take leather, remember a) to let it dry slowly and b) when wet you have an opportunity to improve on the fit by boning it. The handle end of a toothbrush makes a good field expedient bone.