my factory marine power 454 has " 8:0 " compression. more likely 7.8. i always run 87 no problems there.
The ONLY way to know what you need, is to know your compression.
you can also do a compression check to get a feeling for cylinder pressure and work backwards with the cam from there.
and for all those that say "always 91 in a boat" just remember that if you dont have enough compression, the motor will actually perform WORSE with the higher octane.
Second - altitude makes a big difference too as the air, and therefore compressed air is less dense. Thats why places with high elevations offer 85 octane fuel at the pumps.
where are you boating?
