IMO, you guys should be more concerned about running ethanol free gas in your boats over what octane rating your running. I won't get into the havoc that ethanol will wreak on your fuel system, google can tell you all about that.
If you fill up with 89 or better you shouldn't have any octane related issues. I've used 87 in my old sunsport in a pinch and didn't notice a difference. Modern fuel injected engines with electronic ignitions will retard the timing to prevent engine damage if lower than recommended octane is used. Older boats usually have lower compression engines and were designed to run on lower (89) octane.
1988 Supra Sun Sport 454 PCM
1972 Olds Cutlass 455
1998 BMW 750il
1996 BMW M3 coupe
1995 Toyota Tacoma 4x4