Apart from the fact that this word has a ghastly et ending,* swimmeret is thoroughly misleading.
Swimmeret? Is it a young or beginner swimmer?
Or a very small swimsuit?
Photo by Adam Linforth
Or some sort of unobtrusive swimming aid?
Nope on all counts. These:
are swimmerets. See? Those floppy paddle-type things. You find them on shrimps and lobsters and crabs.
Here's a diagram of a prawn with the swimmerets in orange.
If you're talking technically (and who, after all, is on familiar terms with a prawn?) you call swimmerets pleopods. This isn't a great word either but at least bestows a little dignity on the proceedings.
Swimmerets are mainly for, yes, swimming, but they also sometimes carry eggs, as well as catching food and sort of wafting it into the mouth.
Swimmerets also sometimes have their own gills, which in a person would be rather like having nostrils on your kneecaps.
And just think what an effect that would have on fashion.
Word Not To Use Today: swimmeret. Swim comes from the Old English swimman and is related to the Golthic swumsl, which means pond.
*Though I do quite like the word castanet.