While the density change is only ~4% for liquid water between its minimum and maximum (under standard conditions), this can be noticeable if the floating thing is very close to neutral buoyancy.

For example, the Galileo thermometer works by having objects of (relatively) fixed density suspended in a water column. The objects densities are slightly different and such that they are all very near neutral buoyancy, so that the more dense objects will only float when the water is colder and more dense as well. This can be made into a thermometer by carefully selecting the densities of your suspended objects!