Water heater on concrete--should I build a platform? How much heat loss in winter?
Probably won't make any noticeable difference.

Normally the bottom of the water tank portion is insulated if electric, and not remotely in contact with the floor if fuel-fired (there's a burner between the bottom of the tank and the floor, and a substantial amount of space taken up by the burner assembly.) I'd save the idea until you were replacing the water heater, if you want to do it. 

More likely that your inlet water temperature is lower, as well as heat loss from pipes being higher. While deep well water is fairly invariant, city and other shallow-piped water systems do vary quite a bit seasonally.