Ross its already an offense in "boating" which we could be assumed to be classed?
Taken from RYA (I am a member!)
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Alcohol and boating law in the UK
Merchant Shipping Act 1995
Boaters may be prosecuted under the Merchant Shipping Act 1995 if their actions on the water are seen to be endangering other vessels, structures or individuals and they are under the influence of alcohol.
Harbour byelaws
Locally, most harbour authorities have harbour byelaws under which they can prosecute if boaters are found to be under the influence of alcohol when in charge of a vessel.
Railways and Transport Safety Act 2003
A law to introduce drink driving offences (including specific alcohol limits) for non-professional mariners was included in the Railways and Transport Safety Act 2003. This provision has not been brought into force .
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Obviously rather easier to prosecute if you have 30 witnesses to alert the forces!
Moderation is clearly sensible, and as ever IF you have an accident,,,, THEN it gets serious!