seems pretty self explanatory - if you have a boat in Greece that earns its money from chartering, the greeks want a slice of the income in Tax - seems reasonable to me as you should then not have to pay that amount of tax in your own country.
....if you have a boat in Greece that earns its money from chartering.....
and it is a British flagged boat then it must satisfy, and be inspected to meet, both MCA coding requirements and any local country coding requirements. If it doesn't, and it doesn't have the YDSA valid coding diamond visible, then regardless of how safe the boat is the poor punter is risking all by being on a boat that, at the end of the day, is probably not insured.