There are several different types of swim, caused by different events:
'Gay' Swim: This is the non-PC term for an unnecessary swim, for example, if Corran Addison capsized in flatwater and pulled his deck instead of getting t-rescued...
Beating: This normally happens before the actual swim, and involves boat and paddler being thrashed mercilessly in one or more river features. Also referred to as getting 'worked'. When someone takes a properly enormous beating, lasting in duration a really long time and involving materialrisk to life and limb, it might be referred to as a 'rapage'. This is even less PC than a 'gay' swim. Half-drowning would probably fall into the latter category.
Pin: When the boat and paddler get jammed against a rock by the force of the river. This has two flavours, horizontal and vertical. The latter normally occurs on bigger rapids and you don't want to go there. But if you do, it'll make a good pub tale.
Backloop: this is when you fail to clear a feature, and often preceeds a beating. Kind of like a very fast, out of control 180 degree vertical transition... onto your back.
Flat landing: When running a high waterfall - this is what it sounds like and is particularly nasty. Might or might not cause a swim, depending on whether you break your spine or get a compression fracture or break your nose on the deck.
(taken from eusu.ed.ac.uk/clubs/canoe/ )