Siemens Aquaris were tested to 3 feet, that doesn't mean that is the maximum depth you can take them. That being said, I would not scuba dive with any hearing aids, including waterproof ones. The pressure caused by scuba diving may interfere with the seals and other components.
You can always try using a place like Discount Hearing Connection at Best Prices on Hearing Aids - Local Care included - See Prices Now!
Widex Dream 440's are listed at $2400.
Also, here is a review of the Widex Dream 440s.
Review - Widex Dream Hearing Aids - YouTube