Andy -
I see little reason to chuck this and buy a new Sea-Doo. There's only a few possibilities that could remain if the body and seals are intact.
If you replaced the PCB with a new one, and the motor is good (which you indicate it is, based on testing), then it's either the trigger assembly/power switch, or it's the batteries. It _appears_ that all Sea-Doos use reed switches on the PCB, and the triggers are separate, but I don't actually own a Sea-doo to verify this. I just found a parts list online for the thing, and it looked like magnetic triggers.
Seeing as the PCB has been replaced, which implies new reed switches, check that the triggers, which are probably magnetic, both have magnets still in place and haven't broken off, then see if you can use a couple of 'fridge magnets work here to trip the reed switches for testing with the PCB connected.
The other possibility -- have you validated that the batteries in the scooter hold a charge, are topped up, and are capable of discharging at a rate of current suitable for the scooter? Fortunately, these are lead-acid batteries, which are inexpensive to replace if they've headed south.
Lead-acid batteries can suffer from sulfation. If a battery is in a discharged or partially charged state or is never fully recharged for a long period of time, the lead sulfate on the plates can harden and resist conversion back to lead dioxide and sponge lead. This results in a loss of capacity which may or may not be reversible. Also, deep discharges can be damaging (although it would seem the PCB protects somewhat against this...) A 12 Volt battery should not be discharged below about 10.6 V. If it's dropped below this point, it may not be possible to restore it to a full charge, or full capacity (even if you can get the voltage back up...)
If you measure the battery voltage, and it's charged fully, you can do a simple discharge test which will tell you more-or-less enough about the battery state. Get a spare high-beam bulb for a car (55W halogen) and hook it up across the battery. Sea-doo batteries are in the range of 7.5 amp-hours, and usually 12 V (12.6, actually). Clip a multimeter across the battery set to measure volts. You should start out at 12.6 Volts, it'll drop under load, and it'll slowly continue to drop over time.
Measure how long it takes to have the battery voltage drop to about 10.7V or so. If it's working right, should be about an hour and 45 minutes, more or less. If you've got a different size battery, multiple volts (12.6, usually) by amp-hours (your battery capacity). That should give you watt-hours. Divide by the wattage of the bulb (55W here) and that'll give the time that the battery should last, more or less. Handy back of the envelope is all we need to know whether it's reasonable or dead.
The reason we're using a 55W high beam bulb is the Sea-Doos claim to last about 2 hours... which implies that 55W is all the power those motors put out.
Good luck!
