Engines like the Raylar LSM550 have closed cooling systems, so some thought appears to have been taken regarding salt water applications. Over the long run, what stops these engines from corroding into scrap due to salty air and occasional exposure to salt water? These environmental factors are unavoidable in an offshore boat.

I don't know of any other aluminum engines that are used on regular basis by engine builders. Internal corrosion is not an issue with closed cooling. Exterior corrosion would be extremely minimal especially with proper maintenance and the occasional spray with silicone protectant.

Companies like Dart are useing a special new coating on there aluminum heads to counter any salt water corosion. Just FYI

We don't deliver our new Raylar LSM550 engine in raw aluminum to customers. The external surfaces and parts are primed and painted and the colors are customer choice.
Our show display engines were left raw on purpose to get the visual message across that the entire engine is aluminum externally.
Regards,
Ray @ Raylar

Crusader has been using LSx engines for a couple years
http://www.crusaderengines.com/60.html
as has Indmar
http://www.indmar.com/ProductLine/In...-SS/index.html
and the aussies have been using them in marine applications even longer, Mercruiser has really dropped the ball with waiting so long to use this engine (LSx has been out for 11 years now) and hanging on to the original 50 year old design,time to move into this century..
Thankfully forward looking companies like Raylar are taking up the slack.

companies like mercury, evinrude, yamaha, honda, have been using aluminum block in outboards forever. they however drain back the water when not in use. a fresh water cooled aluminum engine would probably never corrode through, maybe surface corrosion where the paint is damaged.

the points of interest will contact areas of dissimiliar metals. iron bolts, s.s bolts and headers. iron solid engine mounts.
proper painting will help for sure , so long as a good etching primer and the proper methods of preping aluminium are used.