It's impossible to choke on a merfolk's cock - only from coming off of it.

Swallowing their cum grants a few minutes or hours of waterbreathing, depending on the amount & how much you had to eat recently (but stay close - it's hard to predict exactly when it will end.)

Eggs woven in your hair also work, but they lose potency quickly if not fertilized.

Being fertilized yourself is the most effective - but I don't think you've earned that yet, do you?