Not one bit of a tool; unless easing a chore counts?

[Toolmaker51]

Not revolutionary or grandiose, barely creative, but........
We have two good sized dogs, 60 and 110 pounds,

with another little 6 pounder,

drink unbelievable mount of water. Often emptying 1.6 gallon/ 6L bottle in just over a day, one of those inverted gravity fed into a bowl type.
Was frequent task to check sufficient level, such as when we are about to leave for a few hours. Contour of bottle ribs and hexagon shape made it unsure to eyeball from a distance.

So, I dropped a spare fishing bobber [floating] in reservoir; white & bright red easily seen even dimly lit.