Manufactured home wiring of hot water tank
The proper breaker would be a 2-pole common trip breaker and not two 1-pole breakers. Buy a proper breaker in the correct current level (amps rating) for the wiring you have and install it. Usually the heater wiring would be #10 copper protected by a 30-A 2-pole breaker. What size wiring do you have? What is the current rating of the present breakers?

Flip the new breaker on and if it stays on, then you have fixed the problem. The tripped 1-pole breaker was bad. 

If the new breaker trips, then the fault is in the water heater. Switch the new 2-pole breaker to off and disconnect the wires at the water heater.

Test the heating element of the water heater to see if it is shorted out. If it is, then you will have to replace the element or replace the entire heater if it is too old or shows signs of impending failure.