I see you are looking for a good used ski, well that eliminates the Seadoo GTR which is a 2012 model and is supercharged and a higher speed ski then you suggested. That also eliminates the Yamaha VXR as you stated you did not want the VX series, which is a bit of a stretch for a three seater and a bouncy ride. I can't comment about the Kawasaki except to say they have the lowest market share and was not one of the brands you liked.
My recomendation is a Seadoo GTX 155, very dependable 4 stroke, big three seater, big storage compartment, great for tubing and affordable as a pre-owned ski. The reason I know so much about this ski is because I own the supercharged version the 2008 GTX 215. My ski runs out at 67 mph, the 155 would run out at 61 mph, it offers a very good ride and handling, we just got back from vacation in Ocean City MD, where we took out my RXT and the GTX tubing, the GTX handled towing one of the biggest tubes made, the Airhead Storm which is a 114 " wide three person upright tube (couch lol) with three people sitting on it , a driver and a spotter with absolutely effortless ease. Now some people will say thats illegal but we were in sheltered waters and had the RXT close by for any problems if needed. with this tube we used a bridle that clipped to the tow/tie down rings so as not to stress the upper single hook. We easily towed it at 30 mph, the turns were a little tricky but no problem. As far as premium fuel goes yes it is recomended for optimum performance but not manditory, most marinas sell 89 octane which is fine the ECU adjusts the timing to compensate for lower octanes especially on non supercharged models. Closed loop cooling is good to have in saltwater. I have seen a post on here about a Yamaha VX with 53 hours and 8 years old have a hole in the engine with impacted salt from sitting with salt water in it, with closed loop cooling that wouldn't have happend due to no sea water near the engine internals, it uses anti-freeze and water just as some closed loop marine boat engines. Lastly when you purchase a jet ski you are not only buying the ski, but the company who made it, and the dealer who services it. Seadoo has one of the best dealer networks available. My GTX 215 has 35 hours on it with ZERO problems, and it is driven hard. I will enclose a pic or two of mine.

By the way that is Yamaha VX cruiser in the background see how much smaller it is. Thats with no one on it...
I am selling my 2008 GTX 215 for 8200.00 if that helps w/ 35 hours. Gettin somethin new next year if it sells. The GTX 155 should be about 800.00 less