Lightning vuln can be covered with gear, or with (very easily dispellable but very easy to get) preps - it's not really a problem. Water vuln can't be really covered except with a handful of items, most of which you wouldn't really want to use (exotic weapons, etc). By itself I'd probably rather have a water vuln than an iron vuln, but I really really hate hydrophobia especially if I am trying to stay aggressive and enemies wise up to moving in/out of water.
I don't think the hp difference between elfs and arials is very substantial?