Penguins.

I’ve been watching “Spy in the Huddle” with the kids, which is, if you haven’t seen it, a documentary series about penguins. The penguins look so awkward out of the water, and obviously they can’t fly, but in the water they move so effortlessly.Here is a link to a clip from a different BBC documentary.
I read somewhere that they can boost their speed by releasing tiny air bubbles from their feathers, which reduces the friction between their bodies and the water. I don’t know much about the anatomy of a penguin, but it seems to me that their efficiency of their movement has a lot to do with their shape and how streamlined they are. As swimmers it is just as important to think about reducing your resistance to the water, as it to think about how to increase your propulsive power.