Question: how many amps does a 50 gallon water heater pull?
Answer: Most 50 amp water heater will come with 4500 watt elements. On a 240 volt circuit that is 4500 / 240 = 18.75 amps, so, no, that will not trip a 20 amp breakers, however, it will be operating the breaker nearly 95% of its 20 ampacity rating and most breakers are not rated for that.