Fish vs water mammals

Okay, don’t call me an idiot for recently realising this, it’s probably common knowledge to most and I am just a wee bit behind the times.

Water mammals, whales Dolphins, seals swim differently than fish. How so?

Mammals have a flat tail which propells them by updown movement.

Fish have vertical tails which propel them by a sids to side movement.

Stupid? For all In know there are mammals that have FISH tails, I just couldn’t think of any, so I thought it was like a epiphany of some sort.