I would recommend a 2002-2009-ish GTX or GTi, make sure it has the 4tec 4 stroke engine, either the 130 or 155. I would stay away from the supercharged models if you want something that is simple and cheap to maintain. I have a 2006 GTX 155 and it goes about 53 mph consistently, but if I scoot back on the seat and crouch down as flat as I can I've hit 55. It's been a great ski. I wouldn't advise a 2 stroke only because it sounds like you want something that you can ride when you want and not worry about having to do a bunch of maintenance all the time. While a 2 stroke engine (especially a 720) can be very reliable and virtually maintenance free, to get it to that point will likely require a total fuel system rebuild to include the carburetor. With a 4 stroke just check the oil each ride, add gas and go; change the plugs and oil once a year.
Plus, with you living by the sea, the 4 strokes only take in minimal salt water for exhaust cooling. They have their own closed loop cooling system for the engine that has its own coolant like a car. It makes the salt water life much easier if you're new and don't want to deal with the hassle of an old 2 stroke that takes its cooling water from whatever water you're riding in.
My last piece of advice for recommending that year range of ski is that they are 3 seaters with manual reverse. While I will admit that I've never rode or played with a ski with IBR (electronic motorized break/reverse), it seems that it gets quite pricey to fix and on a 10 year old ski, you might have to do that sooner rather than later depending on how it was used. I've seen a lot of IBR problem threads here and other sites lately. Manual reverse is simple, pull a lever on the left side and you're done, a cable does the rest and as long as you lube it and protect it from salt, should last a very long time and be much cheaper to repair than IBR if and when it goes bad.