Check the main circuit breaker wires on top of the motor (red push button). Had a similar situation with a newly installed 350 MPI and found the small nuts for the wires to that circuit breaker had not been tightened at all. After tightening them she fired right up again.
Disclaimer: Let me say "I did winterize by the book". No step in my owners' manual mentioned intake manifold plug removal (behind alternator tensioner) during winter preparation.
Situation: Boat stayed at the marina in a lift slip this winter. Sunday before last was beautiful here in East...
