This is one of the downsides of the walk-in spots I tend to hunt. I am pretty OCD about making absolutely sure that I'm in no sort of "gastrointestinal distress" before going in, because if you are 300 yards away from dry land it can pose some serious problems.
In one spot there is a great big beaver lodge that's the only dry place around. It has saved a couple of folks from full-blown emergencies.
I've also climbed into blown-down trees and straddled 2 branches a couple of times. Compared to other situations, that feels like an absolute luxury.
I always recommend having 2 or 3 screw-in steps, or some sort of strap that could be wrapped around a tree, to give someone the ability to step up out of the water.
As far as urinating goes, never underestimate the forehead-lean (I coined that term within our hunting group a few years ago). Put your head against a tree to allow yourself to lean a bit more. It helps to have more "reach" on those especially cold mornings when you are trying to avoid peeing on yourself and your "clearance" isn't as good as it would be in warmer weather