Corrosion is much, much more complicated than one reaction formula.
However, 316L shall NOT corrode in seawater. It's meant for that use, and experimentally 316L excels at this job.
The flaw lies very probably in the epoxy. Upon hardening, most epoxies release hydrochloric acid and more noxious products that do corrode most metals, including stainless steel. This is a design error.
Some epoxies are designed to release no corrosive products. They cost a fortune. The chip industry has one fab in Japan to provide the epoxy that encapsulates integrated circuits without corroding the ultrathin metal pads. To my opinion, the solution in a boat is to use an other material than epoxy+fiberglass, or to the very least, harden the epoxy away from the metal, then flush it for long.
Sacrificial zinc won't help against that, nor is it needed with 316L used properly in seawater. I wouldn't focus on metal contacts and corrosion couples, as in my experience they aren't essential. They do matter with ultrapure metals, for instance in the self-discharge of batteries, but not with alloys, as these contain already corrosion couples at every crystal.