I don�t think the two scenarios you mention are comparable really.
Firstly the guy by the lake, is that any different to him being by the lake in swimming trunks like the little Speedo ones? Or do you think it is wrong for people to wear what is basically underwear, in all but name on beaches/by lakes etc?
The second one well personally I couldn�t care less if you walked down the high street in your underwear, but I can understand why some people would disapprove, time and a place and all that. Legally to be honest I�m not sure guess there is no specific law against it, but if there were complaints I think the police would stop you and ask you what you were doing.