When you say "locks up" is the motor vapor locking of so flooded it physically has too much gas in the cylinder bore?

Vapor lock I understand. The other would be too much fuel and very bad injectors/carb.

Trayson is right though. Spark is your problem. Check for bad fuses, broken wires, or loose connections before throwing money at it. If you don't have a check engine light there aren't codes to check.

One more thing, the ignition circuit runs through the oil level switch and coolant temp switch on your boat. If either are bad or the wires came off the boat will not fire.