What's the best way to swim?

If "best" is the same as "tried and recommended by the majority of fish," then the answer is easy. Think of your basic sardine beating its tail back and forth propel itself forward, and you've got it. This undulatory swimming is used by aquatic animals from the size of a pencil eraser to the size of a house, and it is by far the dominant method of locomotion in the ocean (at macroscopic scales, at least).

But there is one whopping exception to the rule: squid, of course. Although they do have fins, and various species undulate these fins to various degrees, they are only a supplement to the real thrust: jet propulsion. The squid sucks water into its body, and squirts it out through a funnel.