In reply to Bjartur í Sumarhús:
I've only used seal skins for walking and found that they worked very well, I even tested them quite a bit and then went in water above the top of the sock if it's in the right place under tension it can just about hold out the water, probably work for a quick in/out dip when crossing a river by hopping from boulder to boulder but probably to cross a river, take them off and then put them back on with the wet boots, they're brilliant if you use common sense.
For trail running, on the rare occasion that I go, I'll put up with the wet, I guess a pair of toe socks could work well if you're having problems.