A jet ski has a 15 gallon gas tank on average. A few larger models have 18 gallon tanks and some smaller ones have 8 gallon.
A jet ski burns 10 gallons per hour at full throttle and 3 gallons per hour cruising. So you can go 3 hours with mix of cruising and racing.
For comparison, a motorboat uses about 30 gallons per hour at full throttle due to larger engine and weight of the boat.