Sounds like the Throttle Position Sensor is not working. I'm not familiar with GM TPS's, maybe someone else can identify the sensor in the picture you posted.
I found this info on this online Indmar manual:
SPN 65618
FMI 7
Fault Description: ETC Return Fault Mechanical System Not Responding or Out of Adjustment
Diagnotsic procedure:SPN 65615, 65616, or 65618 on page 9-147

ftp://ftp2.indmar.us/Manuals/Service...iagnostics.pdf

From reading the disgnostic procedure, sounds like the ELECTRIC MOTOR that moves the butterfly is the problem, not the TPS that reads it's position.

SPN 65615, 65616, or 65618
Diagnostic Instructions
• Perform the Powertrain On Board
Diagnostic (OBD) System Check on page 9-64
prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis on page 6-4 for
an overview of the diagnostic approach.
• Diagnostic Procedure Instructions on page 6-5
provides an overview of each diagnostic category.
SPN Descriptors
SPN 65615 FMI 7: Electronic Throttle Control (ETC) Actuation Fault Mechanical System Not Responding or Out
of Adjustment
SPN 65616 FMI 12: Electronic Throttle Control (ETC) Process Fault Bad Intelligent Device or Component
SPN 65618 FMI 7: Electronic Throttle Control (ETC) Return Fault Mechanical System Not Responding or Out of
Adjustment
Circuit/System Description
The engine control module (ECM) controls the throttle
valve by applying a varying voltage to the control
circuits of the throttle actuator control (TAC) motor.
The ECM monitors the duty cycle that is required to
actuate the throttle valve. The ECM monitors the throttle
position (TP) sensors 1 and 2 to determine the actual
throttle valve position.
Conditions for Running the SPN
65615–7
• Before the ECM can report that SPN 65615–7
failed, SPNs 65601, 65602, 65610, and 65618–7
must run and pass.
• SPN 630, 65580, 65620, 65621, 65622, or 65623
is not set.
• The ignition is ON, or the engine is operating.
• The minimum throttle position is learned.
• The ignition voltage is greater than 9 volts.
• SPN 65615 runs continuously when the above
conditions are met.
65616–12
• SPN 630, 65580, 65620, 65621, 65622, or 65623
is not set.
• The ignition is ON, or the engine is operating.
• SPN 65616 runs continuously when the above
conditions are met.
65618–7
• Before the ECM can report that SPN 65618–7
failed, SPNs 65601, 65602, and 65610 must
run and pass.
• SPN 630, 65580, 65620, 65621, 65622, or 65623
is not set.
• The ignition is ON, or the engine is operating.
• SPN 65618 runs continuously when the above
conditions are met.
Conditions for Setting the SPN
65615–7
The difference between the commanded and the
actual throttle position is greater than 10 percent for
greater than 276 mS.
65616–12
The predicted throttle position calculations in the main
processor differ from the motor control processor.
65618–7
The ECM detects that the throttle blade did not return
to the default position, during power up.
Action Taken When the SPN Sets
• SPN 65615, 65616, and 65618 are Type A SPNs.
• The ECM commands the TAC system to
operate in a Reduced Engine Power mode.
Conditions for Clearing the SPN
SPN 65615, 65616, and 65618 are Type A SPNs.
Diagnostic Aids
• Inspect for a condition in which the throttle valve
may have been held open. For example, ice
may have formed in the throttle bore causing the
throttle valve not to close.
• A high resistance condition on the throttle position
and throttle actuator control circuits could cause
a DTC to set.
• A low battery condition may cause a DTC to set.
Engine Controls and Fuel - Marine 9-147