Why do most marine mammals have horizontal tails, while other marine life tend to have vertical tails?

Question

Why do most marine mammals have horizontal tails, while other marine life tend to have vertical tails?

In: 75

Answer 1

I’m not sure but I think it’s because marine mammals need to go up to the surface for air, while fish and other don’t. The horizontal tail makes it easier to go up-/and downward. For fish(es?) It is not as important to go up and down.
That’s my guess