Or he just decided not to regenerate and that UNIT chap was talking out of his arse!
Maybe he did regenerate, but just drowned again and again until he ran out of regenerations (or until he figured it was pointless)?
That's what I always assumed happened. We never saw his face under the sheet, so he very well could have regenerated, drowned again, regenerated, and so on until they ran out of regenerations or British celebrities. If you pull that sheet back, it's actually Jeremy Clarkson.

