That is not a very accurate way to measure at all. Your hand is not delicate enough to feel the difference between a few degrees. You need to get your hands on an IR thermometer or at the very least a digital thermometer with a probe to measure the actual temp. Odds are that if your TSTAT is regulating to 90ish degrees then you will have a slight heat loss through the plastic, not much, but slight. However I wouldn't bump temps up until you can verify the actual surface temp of the inside of the bin.
On a side note I say the water dish is fine.