Doug: Given the choices, I'd go with Marelon because it has no galvanic issues and is tough enough to be frequently recommended by the Boat US insurance people.
Yoop: As for sealants, 4200 is a flexible, "non-permanent" choice. I'm pretty sure it's ok to use underwater.
Joel Albert, Potomac MD
"Charlie B" - 1988 32' FBS
Twin 318's/FWC/16x15 nibral props
docked Deale, MD