Battery service by mail
If a local competent shop can replace a basic battery safely, two-way shipping may cost more than the service. Mail-in makes more sense when the watch needs diagnosis, sealing work, a difficult case opening or another repair at the same time.
A dead watch may not have a dead-battery problem
A suitable fresh battery can expose a failed circuit, corroded contacts, blocked hands or a worn movement.
Rechargeable watches are different
Eco-Drive, solar and Kinetic watches should not be treated as ordinary disposable-battery watches.
Water resistance is separate from battery replacement
Opening the case creates an opportunity to inspect seals, but a water-resistance rating should only be represented after appropriate case condition and testing.