Back in the 1800s this Swedish guy Nils Blommér got really good at painting images from Norse mythology and folklore, like the one above, which features a näcken (Swedish for neck, which in turn is apparently the English term for a male nixie) surrounded by the daughters of Aegir. I think it gives us a pretty good idea of the sort of eerie, moonlit recreational activity that the old näcken and certain wave maidens enjoy partaking in. Needless to say, that’s some pretty hardcore strumming that’s going on there.
There unfortunately doesn’t seem to be a good, quality source about Blommér online, but check out these other works of his on wikipedia because they are also worthy of your attention.