weedpharma:
Any excuse to play is a good excuse!Welcome to the forum. Thanks for letting us know your experience, it helps to know what experience you have.
Arduino has many sensors available that can do what you want. It depends on your ability how to put it together.
You could use Uno for development then use a minimalist version using the processor on your own board for the real thing.
You forgot the simplest sensor for door opening....a switch
Weedpharma
Thanks, yeah it seemed clear that Arduino was more than up for the task.
I did consider a simple door switch and haven't ruled it out. I am trying to avoid mounting anything to the inside of the safe, or to have to run too many wires. I'd prefer a mostly self-contained portable solution that could just pick up and be transferred to another safe or cabinet or whatever. Water level sensing will require some external sensor, unless I can waterproof everything but the sensor and just toss the whole thing in the floor.