If your main issue is protecting the pump, perhaps investing in one with its own float (if the pump is an immersion type) or flow (if it’s external) switch is the way to go. That’s the only thing that needs to work at line voltage.
That said, some ideas for monitoring level:
- optical prism sensors, in a dip tube
- Capacitive sensor, used in the RV trade, again in a dip tube
- Pressure sensor
More here: Is it possible to do water detection with a single electrode?