This isn't quite true as I have often been offered 6063-T6 as an alternative to 6082-T6 and this has a much lower yeild strength.

I think that the alloys that Wouter mentions are all good, what you will find is that 6061-T6 is what is offered in the US, whereas 6082-T6 is what is offered in the UK. Not sure about 6005-T5. I was told by my supplier that 6082-T6 is the reccomended alloy for a marine enviroment, this is certainly something to do with its corrosion resistance and could be to do with this alloys propensity for anodising.

Gareth