Does your boat have an MDC? Not sure when they switched over to the managed gauge system but having every gauge go bad at one time would seem strange. When did they go bad? Did it happen all at once? If this started all at once or when you just put the boat in service for the year I'd be checking connections. Almost sounds like you're missing a ground for the gauge cluster (most likely at the battery). Or if you do have an MDC this is a classic MDC issue.
Another way to go at it would be to pick one end and work from there. Pick an easy gauge to work on like the temp gauge and check for resistance at the sending unit. Trace the wire from the sending unit to the gauge and check for continuity on the wires. Then check the resistance on the gauge.
Another thing to try if these are not MDC controlled gauges is to pop one of the gauges out of the dash and connect it directly to the sending unit and see if it works then. That would eliminate the wiring. Hard to believe every gauge went bad though.