1994 454XL starts but immediatly dies and other mysteries.

Went out last week to check the bottom and got in the dirt but got off OK. Anchored for about a hour and when I tried to restart the STBD engine it would start but then die after about 10 seconds. Hasn't started since. Tried again today neither the STBD eng. or the genset would start. Removed the anti siphon valve (new) on stbd tank and the genset started. Nojoy on the eng. Last year I changed the oil pressure safety switch. The electric fuel pump runs, the eng. starts but then it appears to run out of fuel. What is the trouble shooting protocol for this problem? TIA

I'd be inclined to start with that switch. Troubleshooting details will depend upon what you have available...bottom line is to ensure you have a current source supplying the fuel pump during the start and run cycles...probably a good idea to verify the fuel pressure as well, but you'll have to keep the engine running for that check.