How to call a merman:

Essence of Chai
Two charmed seashells
men's underwear
crumbs of sea scumb

Mix together with diluted saltwater, bake beneath the sun, and repeat the incantation spell 10 times. He should appear within the next 30 minuted.

A note of warning: Mermen are very stingy with their time, if you are going to be asking for an extent of time, you will need to be very convincing in your reasons, if you fail to convince him, it may result in him stealing what little time you have left.