What blood alcohol concentration present in a vessel operator under 21 years of age is unlawful?

most boats worldwide now dry. have to adhere to whatever flag of the country they are associated with. if flag of convienience (no country) and outside juristriction of any country, technically no national law to bind it.

in US waters under 21, normal laws apply.

also, there is vessel guidelines to follow and usually these include various drink laws, depends what the vessel is linked to...

this is very rough and varies world over.

used to be lots of drunkenness at sea... still goes on I'm sure but not like it was