When I'm on the VHF
and want to be identified accurately I call myself Sailing Vessel "boat name". That way if someone is looking they look for the stick with the table cloth flapping on it (mast and sails).
I'd ask the owner what he calls his boat, on the radio
and in his log (or check his blog if he has one).
In writing it's (IMHO) S/V BoatName
when you want to be formal. More often than not I leave S/V out unless it's germane to the communication to note the difference.
Note the Italics, I believe that it is the
custom to italicize a boat name.