You can set the timing successfully on land, with the motor not running. Remove all spark plugs and prop. Set motor to TDC using a steel ruler or caliper, thru #1 spark plug hole. Note position of flywheel. It should show close to TDC, Note measurement to top of piston from cylinder head. Back the piston down until ruler or caliper shows it has traveled .464 inches. Timing pointer should point to the .464 mark on timing decal. If not set it to point to it.
Install spark plugs 2-6 into their wires and wire their bases together and to ground. Install #1 plug and put timing light on #1 spark plug wire.
Using a clip lead, jumper the starter solenoid so the motor cranks. Chek timing and advance throttle (in gear) until timing is 4-6*BTDC. At this point carbs should still be closed, but ready to open. There are two bolts on the midsection of the distributor that loosen (5/16" wrench) and allow the idle timing collar to rotate to adjust the opening of the carbs. Set this snugly against the carb lever. That is idle pickup timing.
Advance throttle to full. Check timing. Back throttle down until you get 21* BTDC, if necessary. Tighten the max spark advance set screw (upper right of dist. housing) to stop further advance of the distributor. Alternatively, you may need to loosen the set screw to allow more advance.
Set throttle stop set screw to allow carbs to open fully, but no more.
Install plugs, install flusher and start motor normally. Set idle stop screw (and adjust throttle cable, if necessary) to allow motor to idle at 900-1000 RPMs.
If she idle well in the water, and will accelerate, you are ready to rumble....