Remove crankcase cover.-----Scratch one line on #1 rod cap up to 5 on #5 rod cap.----No need to mark #6 cap.----Need 1/4" 12 point socket to remove rod bolts.
If your " cool looking " prop has too much pitch , then acceleration will be slow.-----Have you adjusted carburetor mixture screws for proper performance , yes , no or no idea how?
The 18 HP motor should make a 12' boat fly at 25 MPH.-----Are you sure motor is in good condition ? ----Properly adjusted carburetor ?----Running on both cylinders?