Surely they could check which boats haven't paid the fees, contact to owners and ask for fees plus proof of insurance which is a requirement. Give them 6 months to pay then remove the wrecks.
Is it time all the delapidated boats in the harbours, most of which are not insured or seaworthy, be removed and scrapped? This will give more space to true users and improve the look of what could be a lovely asset.