Jack,
There is lots of stuff about this on ausfish. Go search for more.
In summary (my version anyway):
Don't use zinc-cadmium or zinc-galv screws/bolts in alumnium ever. Depending on lots of things, they will amalgamate (and dissolve) with the aluminium in weird ways - all bad.
Use S/S (only 316 if below the waterline) and use duralac (a tube costs up to $30 but lasts forever if you are careful with the lid etc) in the hole, the bolt, washer and nut.
If selftapping screws in, first tap them, then remove, put duralac in the hole, then reinsert the screw.
If no duralac, then use marine sikaflex. If no sikalfex and desperate, use neutral cure roofing silicon - but use something - you will be amazed at how fast the hole and surrounding area corrodes (white powder...) and bubbles...and it gets worse quickly, evenutually leading to pinholes around chines and keels.......
Even if you don't use your boat often, you can still watch the corrosion happen in the comfort of your carport (though a bit slower.....).
Note that it is the aluminium that will break down - though the screws/bolts and fittings may discoulor and gradually break down also.
An alternative often forgotten about is to use marine grade (sealed end) aluminium rivets - these are available at serious bolt shops - though even blunnings now carries some. But even then use duralac or something......particularly below the waterline or on the hull.