My A4 (1976) is starting easy and running smooth after several years of replacing ancillary systems (fuel, exhaust, water intake, . . .). Have factory spec compression in all four so no reason to rebuild. Can't see any reason to replace it. When it goes, I plan to replace with a rebuilt or newly built (since new A4 engine blocks are now available from Moyer).
With hundreds of thousands of gas powerboats on the water - recreational and commercial - I don't see a safety problem.
Of course it is always possible that natural gas, electric or fuel cells become available and cheap. Natural Gas is probably the most sensible fuel for a coastal or inshore auxiliary, if only it were distributed to marine filling stations.