Hi Kraut,
I've dealt with this issue a few times before. It's surprising that Tef gel didn't prevent this from happening. Since I started using it I don't have anymore problems even if I'm not unscrewing my screws regularly as I should and as you pointed out.
It's not the salt water that causes this problem. I've had the same issue foiling in only fresh water for a couple of months.
It's the chemical reaction between the aluminium and the stainless steel called Galvanic corrosion:
www.albanycountyfasteners.com/blog/stainless-steel-and-aluminum/ Drilling the screws out without damaging the thread will require some precision machinery. I don't have a reference in Perth but I'm sure you can find someone in an industrial area like Osborne Park who can do it.
Before doing this, it's probably worth trying one of these damaged screw extractors ?
www.bunnings.com.au/frost-5-piece-extractor-screw-set_p6360083Good luck
Christian