Hmm, ok. Sounds slightly strange, when you say it revs up fine on land, I am assuming you're revving it out of gear
Is the cooling on the engine working fine? A nice steady telltale coming out of the engine when it's fired up... Are the oil levels all ok and as they should be? I don't know if this is the case for the BF75, but some honda engines will go into "get me home" mode if they are having a problem.
Other than that, try clearing your fuel filters, and checking that your fuel line isn't blocked at all.
We've pretty much exhausted the Jimbo book of outboard troubleshooting now, and as it is pretty limited, I shall hand over to more knowledgeable forum members!