Fins, Thongs, and Lifeguard Songs pg 1 (Patreon)

One of the signs to look for in a merman in desguise it's his struggle to keep any kind of clothings on. Since they're aquatic creatures they're not used to higher temps, even if tha defies the dressing code when doubling as a lifeguard on the beach. 

In Kai's case, it also helps him in attracting new possible mates and letting them know he's available