1. I would consider it UNSAFE until you discover the problem.
2. It could be a weak breaker, a broken or hot spot in the wiring going to the W/H or in the W/H, a bad thermostat, a bad element, basically any bad connection will heat up which in turn will increase the amp draw and cause the breaker to trip. The first thing to do is check the amp draw on each W/H leg (while the water is heating) at the main breaker panel. Amp draw should be about equal on each leg.