To my understanding the way Popeye describes the seals, and putting water into the seal Tk, there are:
water seal/water seal/stern seal tk\oil seal.
So if two outer water seals fail then water will ingress to the seal water tank.
If the 3rd seal, the after most oil seal, fails then head pressure from the 2 header tanks (whichever is in operation - high tank for loaded) should keep water out.
According to Reed's
Weardown for the white metal should not normally exceed 2mm to avoid hammering out and the period between inspections is about 6 years.
There should be an oil cooler and a circ pump for the system, eh?
...what kind of pressure does the pump produce, Popeye?