Yes, but if they require you to run their proprietary oil they have to provide it to you for free per the Magnuson–Moss Warranty Act here in the US. that's why they almost always have an "or equal".
I ran into this with my Infiniti that only allows the Nissan oil in the transfer case and there is no substitute and they will void your warranty if you use anything else. After calling Nissan and the consumer help line it turned out that even though they were legally obligated to provide the oil, the only way to move forward was to start a case against Nissan. So... it was cheaper to buy the $125 per quart oil then fight Nissan. Yes, I was the only idiot the Nissan dealer ever saw buy the oil and it was covered in dust. I asked what the service department used and they said Mobil 1 gear oil.
Let the fur fly! I don't think there any bad oils on the market anymore, just the wrong type. Personally, I'd use the OEM oil in my 4-tec just b/c it's the correct type, no guessing.
Mobil 1 is good oil, Amsoil and so is Napa labeled oil, even Supertech oil rates well. To me, an important aspect is to change it at least at end of season so the muck doesn't set in the crankcase stewing all winter.
And I don't think the cost of one oil change per year will break anyone's wallet. We have to admit: Sea-Doos are expensive toys to run if you figure all the costs (maintenance, gas, insurance, winterization, etc.).
Here is Canada I calculated my cost at around $50 CAD per hour for a 50 hours season if you figure everything, including depreciation.