I used to run a mini which I did some back end work on, currently own a couple of Boats and a jetski and work within the marine industry where towing is often queried/questioned by my customers so hopefully I'm qualified to offer some guidance.
Couple of pointers I can think of:
- Yes you should be able to fit a tow bar to the cooper S (aftermarket though, Mini do not offer one), and it will comfortably tow pretty much any ski (even upto the big 3 seater yams and seadoo's). You will need to check that any after market tow bar you buy meets the legal requirements of your locality. This will have insurance, liability and ultimately, safety repercussions - if you ain't covered don't risk it; it'd be tempting fate and I've heard some real horror stories of a similar nature.
- I would highly advise against launching with your mini. It's so low that you'll be asking for trouble by putting it near water (fresh or salt). It also will struggle with slipways, beaches and salt corrosion (if you are going near an ocean). if you must launch with it then use a 20ft rope between the car and the trailer. otherwise get it launched for you at a facility that has the relevant kit on hand. Needless to say, do not even attempt a beach launch! hehe
- In the UK, any driver can tow an un-braked trailer up to 750kgs without any extra license requirements - you should ensure that this is similar with you, but I'm pretty sure it will be.
- Buy some cheapo splash proof seat covers and generic rubber floor mats; and have them installed before you go anywhere near the water. It's also be worth carrying plenty of large refuse sacks or big water tight grab bag for your gear - storage on Jet skis is limited and you won't want to be getting your nice Mini all stinky with lake/sea water.
Need to know any more feel free to ask