Im dropping in a 2000 454 efi, boat had a carb 454, but came with no motor. Do I need to change/add anything or will the plug plug in and everything will be honkeydorey??

If it's mercruiser ,drop it in .If not ,what is it

mercruiser, so the engine to dash harness needs no changes, new wires etc??

What year is the boat? Is the harness set up for efi???

boat is a 95, it was a carb boat.

Drop it in plug it in and go!!

There should be a 10 pin plug that will connect all the essential vitals between the dash and the motor. The EFI stuff if all present should be self contained on the motor.