Cover the shank of the bolt body with adhesive-lined heatshrink and use a good thick nylon fender washer under the head of the SS bolt. Grease up the threads well when you put on the nylok locking nut ... and you should be fine.
Galvanic corrosion is exacerbated in saltwater, but I have done many a bolt in or through aluminum this way. I ALWAYS prep the aluminum skin though, using the clean/sand/solvent wash, then zinc chromate acid etch ... but my hulls are painted. At least put a good quality marine grease under the washers that bear against the aluminum, to prevent water intrusion.
Another GREAT tip is to drill the hole through the marine ply 3/8" size, if using 1/4x20tpi bolts, and fill the hole with epoxy. I apply the std consistency, then add thickener to like viscosity of honey, then fill the hole (one side taped off w/ duct tape). When cured, drill through for the bolt body size needed. This leaves a permanent, waterproof 'plug' around the bolt to prevent water and moisture from wicking into the wood core. This is a MUST on any wood cored structural member!