That 1997 420AC yacht likely still has the original Tides Sure Seal dripless seals.
If so, then that is an old design and those parts have been used past the expected service life. Those parts should be replaced w/ current technolgy parts, such as the Tides Strong Seal. Assuming the Tides brand is retained then the (likely) existing spare seal can be retained. When I do the seals on my boat I put the spare as active and the new as a spare (the current seal is discarded).
Yes, it is theoretically possible to swap seals on a floating boat, assuming that it already has a spare on the shaft, but that is not something I would ever attempt.
My 2000 380DA has 1½" shafts. My guess is yours has 2" shafts. The fine people at Tides can tell you exactly what parts are required.
The engines draw in raw water (sea water) for cooling everything. That raw water circuit has a tee fitting to provide these dripless seals w/ pressurized raw water when the engines are running. Again, it is VERY important to perform those two tests annually to ensure this critical system is good.
Sea Ray built my boat w/ dripless seals having a single hose barb for pressurized raw water. When I upgraded those parts I switched to dripless seals having two hose barbs so I could add a crossover hose. That crossover hose permits boat operation w/ only one engine running.
