Water heater breaker gets hot and occasionally trips

If your load is close to the eating of the breaker, such as pulling 14 amps through a 15 amp breaker, it could get a bit warm

However, the much more common reason this happens is because people use the breaker as a switch, which it isn't intended to do. Repeated cycling of the breaker causes it to wear out pretty quick, which will increase internal resistance, which causes heat and premature tripping.