Do mammals lay their eggs in the water?

Of all the thousands of species of mammals, only three lay eggs. The two species of echidna and the platypus are the only egg-laying mammals (monotremes).

Echidnas lay their egg directly into a pouch (really just a flap of skin) which they develop only during breeding season.

Platypuses lay their eggs in a chamber at the end of a burrow dug into the side of a riverbank or creekbank.