If I dropped a bowling ball in a bucket of water which is 45 degrees F, and dropped another bowling ball of the same weight, mass, and size in a bucket at 30 degrees F, both of them at the same time, which ball would hit the bottom of the bucket first ?

The one dropped into the bucket of 45 F water since the water in the second bucket would be either frozen or super cooled. As a super cooled liquid it would start to freeze as soon as the bowling ball hit the water.

There is a point in the temperature profile where density of water is a maximum. This is 4C or 39F, At this temperature the bowling ball would fall the slowest, though unless the bucket was very deep the difference in the time to the bottom would be virtually unmeasurable.

Niel

Reference - Handbook of Chemistry and Physics - http://www2.volstate.edu/CHEM/Densi..._Water.htm