Mermaid pictures by various artists. Click thumbnail to enlarge.
Mermaids have homes under the sea, sometimes castles if they are descended from royalty. There are very few descriptions of these homes, though occasionally mortal men have been taken to them by their mermaid brides. One story I have is about a deep sea diver who discovers a whole city of mer-people under the sea.
When men have gone to live underwater, they have not gone willingly. Usually they are abducted or tricked by the mermaid. You will find several stories in the myth and legend sections about this. I especially recommend "Ruad and the Mermaids".
Notice in the Ruad story, and others, that mermaids are physically very strong - possibly from all the swimming.
In Greek mythology, Thetis offers immortality to her husband Peleus if he will live under the sea with her. Thetis was a Nereid or sea nymph.
Click here for more mermaid pictures.