So I'm just cruising along at 3500 RPM on our way to dinner. Boat just went completely dead. Both motors. No power to any helm switches. Had been out all day and the previous day without a hiccup.

Spent 10 minutes attempting to diagnose before getting a tow back to the marina and walking her back on the trailer. Battery connections are all tight. A couple of items wired directly to the batteries still work. I'm thinking something in one of the battery switches? I did not pull the switches apart yet. Are there fuses in them? Not sure where to start. Maybe lost a major ground?

sounds like you lost ground. Each engine has a seperate power wire to the key for that engine.

circuit breaker on engines? may have a dead short somewhere..

There is generally a main in-line fuse going in to the ignition switch. If that blows you loose everything, gages, switches cranking, everything.

Large fuse that is in your starter. Looks like a little square attached to starter. Your alternator is hooked to it. When they fry it all goes out. Can't be reset you have to replace them. You can check by jump those wires to the hot from Batt.
Hope this helps.

Both engines?

Same question. Both engines?

The toilet flushes (electric macerator directly wired to battery). And the trim tabs work. That's it. Nothing else. I haven't tried putting in on shore power or anything so my house power may still work????

Kill switch........?

How many batteries and how many switches do you have? I agree that is could be the ground, but if you have a single battery or a single switch, your problem could be in either.

A friend just had this happen. twin 496 baja.