One last thing, when you get into the real base numbers of any study, you sometimes need to ask if any loadings were excluded. Studies funded by governmental sources are many times eddied in ways to not implicate, or cost that, or other governmental agency.
For a subject like copper loading you need to go back and ask if all sources of loading are considered and specifically ask if copper from drinking water treatment was included. If you do, you might find that it was not as there are no adequate, and cost effective, alternatives and the copper is added for public health. I’m sorry if I come across as anti-environmental but I have been dealing with the State and Federal environmental agencies for a long time and know where, and when, they burry the bodies.
It will all come down to politics eventually not science. Boats less than 65’ are easy targets with relatively small voting base. Boats, and ships, greater than 65’ get into the commercial area, most are not pleasure yachts. Is your legislation being framed for only non-commercial boats or is it also being framed to include/exclude commercials ships?