yamahas are more reliable than seadoos, yes.
they are basically a jetski, you have to check for rust, compresion, the impeller can be look from outside and measure, but if he have a damage, it most likely will be clearly visible.
you have to test ride it as well. how many hours doest it have?
i think they are even safer for somebody swiming in the rear than an outboard engine, as you dont have an external impeller. you can see that a jetski or a jet boat like the yamahas can go to places with less depth and do less damage.
saying so, you are not suppose to be running the engine when somebody is swiming on the back as the force of the water can produce damage in the human body if goes at that pressure for some of our cavities.