There isn't such a thing as a water proofing service because water proof watches don't exist.
Your watch is probably splash resistant, changing and greasing the gaskets, checking everything fits as it should may make it OK for swimming. A little more work with sealing the crystal, tube etc may make it OK for snorkelling/very shallow diving.
Going past that - 200m/20 ATM WR will depend greatly on how well the case has been made, especially the machining tolerances on the case back/case threads and the area around the crystal and rehaut.