Re: New to me Mariah 1993 diablo few ?'s
 

Isn't boats just a barrel of monkeys! :)
Without being there , it sounds from the description your giving that it is electrical. It almost sounds as thought you either have a grounding or connector issue , or the oil preasure shutoff switch is bad. But make sure your getting fuel.

1) Unplug your main engine harness and clean the connectors real good with brake cleaner then coat with dielectric grease and plug back in , see if she'll run.(Make sure main breaker is OK)
2) Unplug the oil preasure shutoff switch and jumper the 2 wires , see if she'll run.
3) Clean all grounds in the electrical system , see if she'll run.
4) Check to see that you are getting spark at the plugs.

Just some easy things to try before ya get too frustrated with her :)

Larry