You can swim easily if you have the skill.
My adventurer once died in an absolutely stupid way.
The Play Now! adventurer started near my latest fort. The fort was flooded due to a u-bend bug. The entrance to the fort was with ramps, so I gathered I'd be able to just take a dip and go back, to train swimming a bit. Turned out I can't, without swimming. Deciding not to let go, I waddled, drowning, to the only nearby unflooded room I knew existed - the walled-in trade depot with the corpses of the starved elven traders. I unwalled it shortly before the great flood, and placed a door there so I could lock it, just in case. This was now my chance to salvation. I entered the room, letting some water in. Having done so several times, until the room was up to 4/7 in water, I started running about, training swimming. The poor creature I was playing (drider) was almost starving by the time the skill raised to Novice. This was the last chance. I opened the door, and started to swim, without drowning, back to the entrance. Already winded, I get to the entrance, and see... a solid wall of ice.
During the two years that fort existed, not a single tile of water ever froze for any length of time. It just had to happen so, that the drop of temperature at night was sufficient to freeze the ice just at this particular time. The ice melted half a minute after I drowned.