I agree on jet skis.
They may be great fun for the rider, but everyone on the beach has to endure the noise and the stench.
And they sometimes kill swimmers.
Beaches were nicer places before that invention.

Chewing gum is another invention I wish could be uninvented.
It serves no nutritional purpose, is difficult and costly to remove from pavements, and nasty when it gets in a kid's hair or clothes.