Depends on what you mean by breathe underwater.
No mammal can breathe unaided underwater.
Us humans can kinda-sorta breathe underwater by using technology to bring some surface conditions(=breathable air) with us into the depths.
We have to use snorkels or Scuba gear, but we still win.
the blue whale
The blue whale is the biggest mammal.
The biggest mammal in Lake Orion is a deer
No, I believe the blue whale is the biggest mammal.
No, the Blue Whale is reputed to be the biggest mammal in the world.
Armadillo
They do not breathe underwater. They hold their breath under the water and surface to breathe.
whale
I think the biggest animal in the world is the blue whale.
This is a guess: hippopotamus
The blue whale is the biggest of all animals, and a mammal.
u either get fish which can breath under water Birds which can fly or mammals which can do none of the above