There is also a bug where some randomly generated mersims don't have a spesific trait, that gives them the hydration bar, and they are missing their tails.
But to discover the premade mersims, you become friends (of course, what else) with them and ask them. You'd need a pretty high relationship (not sure how high), otherwise they will deny it. After that they will swim in their merform. Also, if one of them is doing their "dolphin" sounds, you can have your sim ask about it. You'll have to be in the right place at the right time.
Not sure if the rest of the mersims are "discovered" if you play a mersim yourself. At least I think the premades do.
