Absolutely possible but would need some modification, if you dont have electronics or coding knowledge it might be difficult. Im working on something similar myself.
The existing ultrasonic sensor and opengarage+ isnt waterproof, so your probably going to have issues eventually with it. Even if you protect it from the weather, being so close to a water tank is going to mean high humidity which will eventually cause corrosion and failure.
You can get a waterproof sensor off ebay. You could then either pull apart an opengarage device and swap it over (soldering required) or buy a microcontroller and program that with the opengarage firmware (much cheaper this way).
You would then need to modify the code a bit to get the additional features you specified.
If youre interested I can provide more detail, but given its been so long since you posted, youve probably moved on.