Harry's Swimming Skills?

[happyjoeysmiley]

No: HPFGUIDX 184132

In GoF, we are told that Harry does not know to swim. (Or were we told 
that he cannot stay *under* water for long?)

Yet in HBP when he goes along with DD to get the locket Horcrux and in 
DH when he tries to get the Sword of Gryffindor, he does swim and gets 
under water.

Just curious if this was a miss by JKR or something which I missed to 
gather while reading all those books. Has this has been discussed 
earlier in this forum / answered already by JKR? Any pointers?

~Joey