Battery, wiring, starter, in that order. First thing check the battery, test it under load, put your test leads on the battery terminals, crank it over without the spark plugs, if the battery voltage is less than 10.5 volts, replace the battery. If the battery is O.K., check the battery cables for corrosion also check the cables, if you twist the cables and you hear a crunch it's time to replace the cables, also check the grounds on the cables, especially where it attaches to the motor. If everything checks you probably need to rebuild the starter.
BTW, hows the 68/69 Seadoo project coming? We would like to see some pictures.
Lou