There was a place in D2:ED where you could drop under the water, as well, if you swam to the right spot (along the path leading to the fjords) and tried jumping to get up higher. There was a dark colouration in the cliff that, from above, made it look like it was possible there could be an alcove or cave below. I discovered the method of getting under the water level trying to climb part of the bank, or at least get high enough to see if there was anything there (it took a couple jumps, but I was able to pause the game at the right height during my descent to determine the dark colour was just rock, and not shadow from an overhang, concealing some hidden treasure).
Walking towards the bank and/or jumping was easily enough to get back to swimming at the surface (ie whenever part of the character got up to water level). I walked along the riverbed, as well, and discovered some of the boulders in the river are not very well supported...
The game should teleport you somewhere safe if it thinks you are stuck. Maybe it can not always handle the situation when the character is under water, but in D2:ED I got teleporter a fair number of times trying to jump repeatedly to get onto or through something.