I found that going out on deck with a phone and pretending to video seemed to modify behaviour. Apparently randomly shooting ignorant twats is frowned on, which is a pity as it would solve some problems.
Licensing is the knee-jerk response and I don't think it would work. My suggestion is that when a jet-ski changes hands, new owner needs to sign a declaration that they know the rules and will be responsible for monitoring compliance. Then at least there would be an avenue for civil action rather than expecting non-existent harbourmasters to enforce the rules. A few high-profile flogg