Clearly you have it wired wrong, or some sort of power wire is rubbed bare and touching the starter circuit.
Disconnect the red/yellow wire at the solenoid. If the starter still spins, the trouble is likely in the motor harness itself. If the starter stops spinning, the trouble is in the boat wiring harness.
If still spinning, disconnect the battery feed to the starter solenoid. If the starter stops spinning, their is some voltage being sent to the solenoid from somewhere on the motor.
Remember the starter has a simple circuit. The battery is connected to one side of the solenoid. The other side of the solenoid is connected to the starter. A lot of motors use the starter solenoid battery side as a connection for all the hot cables. Your trouble may be found there by disconnecting each of the those "hot" cables one by one, until the starter stops spinning. That one will be the culprit.