I did the Royal Navy dunker course at HMS Dolphin (as a civvy travelling out to Grey Funnel Line ships) many years ago and I can heartily concur with Thomas Coupling - it is an experience which you will never forget. Four dunks, last in the dark with darkened visors down: wait until you're upside down (less chance of the 'fish bait' scenario, Paulo) then exit through a defined exit in a set order. Scary. Having 'died' twice, I'm none too happy about helos over water ever since (not good for a North Sea wannabe!!
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But, it has focussed my mind on getting out of anything in an emergency. All these years later, whenever I get in an aircraft (airliner or light aircraft) I make damn sure I know how to get off it again in a hurry, operate emergency exits, etc. Some years after the dunk, I got on an airliner with a colleague who'd been on the same course as me. Just after we'd sat down I asked him where the nearest emergency exit was. Without looking or hesitation he pointed over his shoulder at it saying: "there and I'm going that way". He, like me, had automatically, nearly subconsciously, looked as soon as we'd found our seats. The training worked.