Hi today I was asked the law in relation to the use of a Jet Ski. I said I will look into this. On the Government website, I spotted this. Hope this helps. 

Including link: https://www.gov.uk/
"New legislation will grant more powers to prosecute people for the dangerous misuse of watercraft, such as jet skis.

From:
Department for Transport, Maritime and Coastguard Agency and Baroness Vere of Norbiton
Published
18 January 2023
This was published under the 2022 to 2024 Sunak Conservative government

anyone riding a jet ski recklessly or causing harm to others could now face up to 2 years in prison and an unlimited fine, thanks to a change in the law
the introduction of new legislation follows growth in the use of watercraft during the pandemic
Maritime and Coastguard Agency to be granted more powers to prosecute, as government works to ensure the UK’s waters continue to be some of the safest in the world
New legislation is being introduced to crack down on the dangerous misuse of watercraft such as jet skis, with the Maritime and Coastguard Agency being granted more powers to prosecute perpetrators of accidents.

The new law will come into force on 31 March 2023, before the busy summer period and will enable watercraft users to be prosecuted and bound by the same laws that apply to ships in order to help to prevent accidents.

This follows a boom in the watercraft industry during the pandemic, with the number, size, power and availability of watercraft like jet skis increasing, and their use in UK waters rising significantly.

Today’s (18 January 2023) move by the government will help ensure the UK continues to have some of the safest waters in the world.

Maritime Minister, Baroness Vere said:

The watercraft industry is thriving and it’s great to see more and more people enjoying leisure activities. However, they must do so safely.

That’s why we’re introducing a new law to crack down on any dangerous misuse of watercraft like jet skis. It will give the Maritime and Coastguard Agency greater power to prosecute those responsible for causing accidents or entirely avoidable tragedies.