as long as your sacrificial anodes are in good shape and kept in good shape, the salt water vs aluminium issue isnt as huge as people think because the salt water isnt "eating" or "dissolving" the aluminium per se. Salt water causes the "corrosion" of aluminum by facilitating the creation of aluminum oxide. The salt does not directly attack the aluminum, but causes an electrochemical attack like a catalyst that results in the corrosion. It would take years for significant corrosion to occur unless you just disregarded any and all normal "todo" things" when operating in salt water. Salt water is going to harm everything else metal on the boat way way way faster than the engines and jets. Metal grab rails, nuts bolts washers, snaps, upholstery.. all of these are in more danger from salt water than the engines.
as far as open vs closed cooling..... for decades outboard manufacturers have exclusively used aluminium and open cooling. Im willing to bet they know far far far more than anyone in here about aluminium, salt water, and cooling.... and they are still using the same things for decades with gold star ratings.... translation, its not the issue we all think it is.
Now your trailer, that is a completely different story and it WILL dissolve quickly from the salt water, so quicking there has been case after case of the rails looking perfect, paint looks new... you touch it, the paint was the only thing solid left!!! lol
Rinse boat off after every use and at least 1-2 times a week, and enjoy.
(not to be an ass I say this purely out of trying to save you and your wallet..literally... im just going to say what everyone in here almost is thinking but doesnt want to be "that guy".... due your diligence on rotax motors. There is a reason the nicest thing ive consistently heard them called is a pos. Sure there are die hards... doesnt discount the endless posts rants threads of major engine failure often. With rotax its not if its when. You might get the best one ever made and zero problems, statistically, you won the lottery. If you want any example, hit any forum with boats/skis using those engines and look at the engine problems left and right, now hit a yamaha boat/ski forum and find 3 if that! Note how many pages/threads it took for either... that says a lot)