They are many places to hire a jet-ski, they also offer tube rides, banana rides to name a few, almost all coastal areas will have a facility, just google search the location you intend to do this activity.
The most common injuries sustained are from collisions with other objects and from falling off the jet ski at high speed. The best way to avoid injury is to wear a fitted life jacket. Often in the case of an accident, a life jacket will keep an unconscious or semi-conscious person afloat in the water. There are engine stop devices that are designed to cut the motor in case of accidents, such as a languard attached to the wrist. Always stay vigilant of obstacles such as swimmers, fallen skiers, rocks and boats.
Another common problem is sunburn, sunstroke and dehydration. Jet Skiing is a strenuous physical activity and takes place on salt water in direct sunlight. Be sure to stay hydrated and wear sunscreen with a high SPF.
You are required at all times to wear the appropriate safety equipment, for example, protective clothing and/or suitable head protection.
Note: Check that your travel insurance covers you, this is classed as an extreme sport. Most operators do not pay for the correct insurance to cover their liability so, do so, at your own risk! I have known a people that have had few issues with operators, over-charging and cheating customers, changing the price previously stated on the website or email, know how long the ride is when you pay and make sure you keep time!