Yamaha's and Kawasaki's will last you a very long time if properly taken care of. I grew up in San Diego and my 2 jet ski's saw nothing but salt water for most of their lives. My stand up has anywhere between 125-150 hours on it with probably 90% of those hours being in salt water and I have never had an issue with it. My Ultra 150 has 42 hours on it and I'm going to guess that maybe 28-30 of those hours are from the salt water and I still have no issues with it either. As long as you flush the jet ski out with hose water afterwards and "Salt Away" you will be good. Then wash your jet ski with soap and water and rinse out the engine compartment real good. Then when that is done, WD-40 then engine compartment and all moving parts like your throttle linkage and so on and you should be good to go.
The only reason why I will look at those 2 brands is because they have been around the longest and have the best technology and are the most reliable. Kawasaki has been making jet ski's since the mid 70's and Yamaha since I believe the early 80's. All of the other brands did not come out until at least the late 80's or early 90's and pretty much had borrowed technology from the older model Kawasaki and Yamaha's. I had a good friend back home that was a jet ski mechanic and he was sharing with me all of the problems that he sees with all of the other brands but Kawasaki and then Yamaha were the 2 makes that he would least see in for repair but also at the same time, when they would come in, those manufacturers were the easiest to deal with and get approval for warranty repair work. I too can testify to this when my stand up had a engine seal go out and it filled up with water and sunk at the Colorado river. We refloated it and towed it back to shore but the electrical system was shot and some other things had to be replaced too. The bill was over 4k do to a majority of things being labor costs but Kawasaki paid for the entire thing. Needless to say, I have always spoken very highly of them ever since then!