Where are you going to look at it, can they give you YDS print out, it could be as easy as going to PWC to hook up the computer, Do your best to get an on the water test, if he has cavitation/pump issues it will sound good on the trailer but only in the water will you know it's working correctly. Can you forward us a link to the ad ?
as far as water in the ECU's, I could be wrong but this is what I understand it to be
the 04-06 engine hatch design is such that water outside will run down inside the back engine wall and pool on the ECU box eventually seeping in,
07-09 I think the only water from above issue has been Bronze10 and he had to stop under a gas station overhang that dumped a tremendous amount of water directly onto his boat and not positive if this was water coming down the back engine wall or actually water rising in the engine compartment,
The only way you can check is to remove the ECU and as I understand it that's kind of a big deal, so much that even being a little paranoid myself I'm not going to bother with it.
I think the major symptom of the bad ECU is the fuel injectors not working properly so if he ran the boat for a while since the last oil change maybe smell the oil to make sure you don't smell fuel,
some have put a layer of silicone over the ECU seam to help seal it,
honestly I wouldn't be that worried about it on an 07-09 boat, I'd be more concerned about #3 spark plug corrosion and compression test,
The link for common issues in that boat is in my signature below, good luck and let me know if you need anything,
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