Used to have jet skis a few years back.They are great fun but they cost a LOT of money to run and maintain.
If you buy Seadoo then you need to use the oil they recommend which was £38 a litre back then.To put it into perspective,i used to have a XP Limited(1000cc) which i tried to get out on most weekends.I used to run the bike as much as i could over the weekend as i had to travel to a lake.It uses a full tank of fuel in around an hour if you run it flat out or about a couple of hours if you mess around on it.A tank of unleaded cost around £34 and i used to take 4 full jerry cans with me which i used to just about get through in a weekend.The bike would also go through most of its oil as well(will let you do the maths).If you get an older bike you have to mix the oil yourself.
If you buy Yamaha or any other bikes,you can pretty much use standard oils which are a lot cheaper.I used to own a GP1200R which is a quick bike,good for towing tows or someone ski-ing.The difference between the Yamaha and the Seadoo is the hull shape which makes a LOT of difference to the ride comfort.Most Seadoos have a semi V shaped hull which will cut through chop easily without any stress on your arms,where as the Yamaha hits chop like a brick wall and kills your forearms.This mainly applies to going in the sea with them which is another story altogether.
If you take them into the sea,you MUST flush them out and give them a good cleaning ASAP or they will corrode internally.If you just use in freshwater,you can just pull it out,empty it and put it away to be cleaned whenever you choose.You may say you will clean the bike after being in the sea but i hated it after i was knackered from fighting waves all day.If you need any advice or help,just gimme a shout.I tried a little bit of racing in the 1200 runabout category and its manic and expensive(hence my short lived try).
All this aside,i think jet skis are the most fun you can have.I have done motorcross,trials riding,riden superbikes and tried most motorsports and i think the jet skis are awesome.Everyone can have a go and its a great weekend out.I will be getting another one next year hopefully and canr wait.