Re: 2 jetskis...worth the buy?
Sea Doo's came out about 1989, and a bunch of improvements were made in 1990. Both of these machines are old technology ski's, but were the best available PWC's at the time.
I bought my first Kawasaki Jet ski in 1984, and later bought a 1990 Sea Doo 587 2 seater that still runs great. I attribute its good service to not running it to death--and using good ole WalMart outboard motor oil. This machine will run about 47 mph, and it's pretty nimble and enjoyable to ride.
I would imagine the 1993 STX 3 man PWC would run 42-45 mph in great running condition, but it's a pretty boring machine to ride by modern standards.
I had a later model SeaDoo XP, with the 787 engine--and it was pretty fast @ 55-57 mph. I have had no experience with the smaller engined XP's of earlier years. I would imagine it'd be good for about 50 mph or so--in good condition.
Sea Doo later used much larger 2 stroke engines, and eventually fuel injected them. They were pretty nice and could be modified into fire breathing speed machines--see Sea-Doo.net for Perry Performance's take on the 2 stroke Sea Doo's.
I bought a Yamaha VX Cruiser last year--the largest selling PWC on the market. It's highly underrated @ 110 hp, and it'll run 54 or 55 mph on top end. The new machines are fast accelerating, fast cruising, and I get 10 mpg. Needless to say, the ride, comfort and economy are big improvements over old generation PWC's.
The only drawback on the new PWC's is they're now 800 lb. machines (with fuel), and they're actually hard to push around your driveway on a trailer. I ended up adding on to my boathouse for my Waverunner, as the manual dock lifts are $1k and electric dock lifts are $2K. They're too heavy for a davit boom lift, and too heavy to push around with a rolling cart on a dock.
2 stroke PWC's can be just a big hassle unless you're a trained mechanic. Parts are very expensive, and paying $110 per hr. mechanics to work on them runs into too much cash outlay for the return. I'd be looking for one late model, low hour 4 stroke PWC if I were you.