Its a GLWPTR and looking thru the WT manual at boatinfo list it should be an EST distributor. That being the case you need a timing shunt to plug into the distributor port and connect wire to 12V. Start the motor and set timing. Once timing is set, shut motor down and disconnect timing shunt
I'm unable to find the timing spec in the manual but GM/Merc has it set at 10 BTDC