What's this?

It's on the transom, under the swim platform. The small metal plate is loose. Not sure what it is and what purpose it serves.

Looks like your transom zinc to me. It might work better if it was tightened.

I agree that is the main zinc.

Thanks guys!

While you are on the hard, pull that transon zinc off and clean the mounting thoroughly. Sand paper and a mini S.S. wire brush for the bolt and nuts and clean the mounting area on the zinc itself. I've found even new zincs need a cleaning of the mounting area. They look bright but the contact area cannot be trusted.

I learned the hard way that not checking and ensuring the contact area is clean is a sure way to a non effective zinc, which you don't want to find later.

Check the connections on the boat inside where the mounting bolts come through the transom. If these connections are in poor shape the zinc will not be effective.

If that is attached to fibreglass then it is of no use
It needs to be attached to metal and the contact surface must be clean
Do not paint over zinc anodes

Allan