I would have started with a compression check and spark test before I did anything.
I would run the diagnostics.
Tall Sail Marine
coils on the HPDI run off the separate injector driver, they require 90 volts due to the excessive pressure.
could also be a main power relay.
you are going to need a factory service manual with wiring diagrams.
www.teamcharlestonmarine.com
www.bombislandboats.com
IF I RESPOND IN ALL CAPS, ITS NOT ON PURPOSE, AND I AM NOT YELLING