Water heater breaker gets hot and occasionally trips

With power off, check the tightness of all wiring in your power center including hots, neutrals, +12, and grounds. It is not uncommon to find screws not properly tightened. And a loose or poor connection can cause heat, especially on a high current circuit. You can also replace the breaker with a new one of same type and amperage. They can go bad. I had one on an air conditioner that would trip occasionally. Replaced it and no more tripping. Inexpensive and easy to change if you are a DIY type.