My SportJet powered SeaDoo X20 seemed to have a mildew problem in the engine bay, and Simple green and a rinse got rid of most of it. Good to know!
The components in the engine bay are sealed, but I would never go crazy with water cleaning it. As mentioned a pressure washer at distance uses less water than a hose, and the distance takes the impact off. I'm talking about 2ft and I can hold my hand in front of it. Then coating the engine with a silicone spray (the same used for the cleanout plugs) will keep it looking nice.
Don't be afraid to pull those cleanout plugs to be sure they can be moved. (I think that was already mentioned) You should be able to grab the jet nozzles and move them from side to side without moving the steering wheel. The reverse buckets should come up off of neutral without shifting inside as well. That's the common amount of play. im glad you mentioned as i forgot. when i went to go pull the clean out plugs, they were full of water and the owner said dont pull them as they are a pia to put back. so i dont think he has taken them out for awhile. with them full of water and not being removed for awhile, what is the worse case with them?
Open all cabinets, and give it a good sniff. Likely to be mildew in other areas if it was not covered. i checked all cabinets and they didnt have any smell. a few seats had mildew spots that we plan on using marine 31 on them. The bimini looks great, if that is a current photo. according to the owner it is new. Be sure the boot is included so that you can stow it. glad you mentioned that. will have to ask. if not, can i tow it open? Also check for the little seat bolsters for the bow, and the two bow filler cushions. those are all there and they look new!
Check the radio controls at the dash, and at the stern. both work. has two amps, a sub, and i think all speakers were upgraded and all work. Check the docking lights, bow nav lights and stern light for operation, and that it is there. checked and all work. Be sure the engine bay fans operate, check oil levels etc. fans work as they should and i checked oil. oil looks good and really clean.
O the trailer, check roller, tire/wheel condition, winch strap and winch operation. If you want to test the surge brakes, unplug the lights and back it up something. The brakes should engage. (mine did the other day with them plugged in for some reason) trailer does show rust and wear from salt. the worse i can see are the straps in the back. (cant even use them) and the leaf springs are showing rust. will check the winch when we put in water.
Bunk condition is nice to look at when the boat is off trailer, but those are an easy fix and not a deal breaker. Look over the hull for scratches below the gelcote level. Around the anchor locker from the bottom side is common, as some let their anchor bounce up there. The keel is a clear indication of abuse. i didnt see any major hull scratches deeper then the gel. i will once it over again. anchor locker looked good and i didnt see anything to major. the ladder is missing tho
Good luck!