I agree with Gilberts on everything except the maintenance part.
Yes if you are experienced enough to service your Vehicle then you can do a lot of the routine things required on a Jet Ski.
Trouble with a Jet Ski is do you have the gear to be able to siphon the oil from the sump ?
Do you have the cable lube kits do you have a Map TunerX or similar to make sure the Ski is running correctly.
Jet Skis are like a modern car, mostly all electrical.
My Jet Ski as most have wireless acceleration, no actual cable to a throttle body, how do you test this at home ?
How good would it be if you did your own maintenance and then one day you fall of the Ski and because it wasn't serviced correctly the Ski keeps going, or you allow a less skilled person to ride and the accelerator sticks full on ?
To many things that can go wrong if you don't know what you are doing.
Oil change and Filter change fine and simple lubrication of the easy to get at cables and connectors.
A mechanic or Shop will know the things you don't on what does need to be done in areas you can't see.
YouTube is full of mainly morons that think they know what they are doing and think they can make money showing people how to do this or that.
I can tell you this for a fact from my expertise in my fields that 90% of videos I watched were useless garbage.
Because 98% are not experts they don't know what they are talking about and end up costing you more as they for one don't have the right tools and are not skilled enough to be able to tell you what will happen if you make a mistake that you don't even know you are doing because they didn't know themselves.
For the sake of a coupe Hundred dollars every 50 to 100 hours it's not worth the cost of an Engine or Drive assembly, or some ones life.
I have seen first hand what a poorly maintained Vehicle can do and if you would have asked the person that owned the car they would have told you it was maintained well as they did it themselves .
At least on land if your car breaks down you can walk to the nearest Servo. Or call some one for help, hopefully you didn't crash because the brakes you fixed failed or a wheel fell off because you didn't know how to do the wheel bearing correctly or 1000 other things that can go wrong because you skimped on doing something as you didn't have the right tool or time.
You could be 6K out at Sea very quickly on a Jet Ski and if it sinks on you because you forgot to reattach a hose or a clamp or you didn't realise that you loosened something whilst doing something else or many other things that you could have accidentally done whilst saving yourself all that money and then you have the fun of trying to swim back.
I have towed many a self maintained Ski back to shore as well as helped to save a Ski from sinking a few times, so don't try and tell me it doesn't happen.
I am a skilled mechanic and I still take my Ski in to the dealer as I get a warranty for the work done and my Insurance covers me for a new Ski if it sinks or blows up as I got the Ski serviced regularly by a Licensed mechanic.
Try to get the insurance on a Ski that you serviced yourself
If you can't afford the $300 a year or every 6 months depending how much you ride then you can't afford to buy a Ski in the first place