Well, in this case I would simply say that you shouldn't be wandering around on water squares; nobody should. You tend to get ambushed and die. If you need to travel over deep water, use the "lower" dungeon-style map. It's slower, but much safer.
My survival tactics at higher levels largely amount to becoming succesively more cowardly with every level. Not sure if I can handle a group of monsters? Genocide them. Big summoner? Teleport it away. At later levels, I try to fight an exclusive group of "familiar" monsters that I know give good exp and treasure with little risk (that is, mostly Great Wyrms, druj, things like that), although I also fight uniques with vigor (anti-summoning tactics are *crucial* here).
So strange as it may sound, the more powerful I get the less aggressive my tactics. When in really dangerous situations, I use the 600 hp rule... source says that nothing can hit you for over 600 hp in one turn (excepting basic elemental attacks and poison), so if you keep your hp around this level, you're much more apt to survive (there are lots of caveats with this system. but you get the picture).
Having a Necromancy skill above 30 is also very very helpful.
Nowadays the only high level chars I lose are on special levels where I can't teleport or genocide or anything. Excepting these situations, there's about nothing you can't prepare for in the later levels of ToME.
But stay out of the Nether Realm. It's for your own good.
