The UU code stands for under voltage shutdown due to the genset being overloaded. Not likely a result of genset's engine problems ... check for the overload condition first.
Switch off boat's 120vac genset breaker (generator supplied power cut off) and restart the genset under no load condition.
If that works, switch all 120vac systems off, switch 120vac genset breaker back on and start troubleshooting each system individually. When all systems check okay, then start increasing the load by switching on multiple systems.
The water heater, a/c, and microwave or stove running concurrently can take the 5kW genset down ... especially if you keep them powered while the genset is being started.