I have a friend with a 28' cruiser, 350 mercruiser, Bravo 3 and it is totally factory. Last weekend he lost all electrical power to the whole boat...I mean it was working one minute and the next it wasn't...no buzzers, lights, etc. We reset the breaker on the ignition with no change and checked the batteries and scratched our heads with no luck.
Does anyone have any ideas or suggestions? Any help is really appreciated.

battery switch? I dropped a wrench dead on top of my switch one day, and boom. no power. I tapped the top of the switch and its back. check power to the switch and then out of switch.

Also try moving or disconnecting the big multi wire plug at the rear of the engine and make sure it's tight.

And if those check out then check the main ground cable for looseness or corrosion

There is a fuse on the starter, looks like a little square box. The battery cable goes directly to the starter terminal and this connects the rest of the engine harness to it.

Get a test light and poke around, start at the battery with the light clipped to the battery minus to check the positive circut. Go to the cable end at the starter, then the other term on the fuse.

Then clip the light to the battery poisitve and check the ground side - just probe the block someplace to verify it is grounded.

It could be any of the things Ted , Wally & Twin suggest.

This will at least narrow it down.

Thanks to you all for the help...we'll check'm all first thing this weekend. Get back to you after to let you know what we find.
Thanks