“Number one. I’m not a prostitute. My occupation is far more complicated than just sleeping around for some extra cash – I have a ton of that already.”
There are two parts of his occupation:
1) “Is the water good?” (or, according to 4 Minute, “Is it poppin’?”)
This is a saying used in Seoul’s nightlife asking about the number of good-looking people at the clubs. The more good-looking people there are, the more people will try to get into the club, hiking up the club’s worth. Jayce gets paid to make appearance at clubs, to make “the water” better, thus transforming the club into a more elite one.
2) “I’ll be your be your boyfriend until this song goes off”
Club dancer. Escort. Call it what you want but there is no specific name for his occupation. People pay him to dance with them at clubs for mostly reasons like… Making their significant other jealous. In the simplest of words, he’s a rent-a-dance-partner.