I have a pair of 2003 Bravo 3 drives and am replacing the shift cables in both. I would prefer some additional insurance that such reassembly is leak free. Could anyone recommend some sealant (silicone or an alternative) to coat the rubber O-ring that seals the upper cover-plate which covers both the upper gears and the shift linkage compartment as well as the cable fiber washer on the other side? I previously had water intrusion in both shifter compartments from the opposite cable end. Thanks...

I use the Merc Perfect seal on all orings.

Yep, Merc's perfect seal is the stuff..