Biggest issue is that the battery may not be expensive to replace. It's is the owner mechanically handy to do such a replacement themselves, without damaging the circuit boards and other internal parts? Being a waterproof unit, more of a chance of damaging seals and or no longer having a unit guaranteed waterproof after a self repair. Countless detectors have been ruined by backyard mechanics and rendered useless that no manufacturer would honor as a warranty repair, nor should they.
To make sure, the average Joe will not damage the Simplex. Nokta pretty much made sure, you can't get into the internals. Factory battery replacement will guaranteed the Simplex will come back as a sealed waterproof unit.
By the way, if the Simplex was not a depth submersible waterproof unit, more than likely a screw on battery compartment door like a cellphone, would have been added for easy battery replacement.