In terms of rot, the only water you need to worry about is that which sits around the post, where the post enters the dirt.. That's where aerobic bacteria lives, so that's where rot happens.. Below, say, six or so inches of packed dirt, things don't rot.. And, before anyone can flame me for saying such a thing, think about silage.. Same deal. No light, no air, no aerobic bacteria, no rot..
If the water in the hole prevents you from tamping it solid, use concrete toward the bottom.. If the high water table prevents water from draining off the post near the surface, throw some gravel in near the top to help with drainage.. Should stand a long while that way..
Or just pick the hole up and move it to another spot, where there isn't a spring. :lol: