Unfortunately your boat is suffering from stray current corrosion
... a very, very common problem. There are 4 things you can do to protect your boat.
1) Install a galvanic isolator
(if you do plug
in to shore power)
2) The Grouper anode is a great idea as it adds much more anode surface area to your boat but a temporary solution only. The anode is simply an extension of your existing cathodic protection. You need to connect the click of the Grouper anode to a metal part which electricalyl conductive to what you want to protect. For example: the prop is electrically connected to the outdrive/outboard, connect the grouper clip to an non-coated surface such as the engine
mount bolts. Or if your boat is bonded, connect to the bonding strip (best). Do not connect to your battery
3) Continue to inspect your other anodes. The Grouper size and mass will definitley help but the smaller engine anodes will waste much faster. Do not mix anode alloy types. OEM anodes are typically aluminum
allloy. A Grouper found at your local store is typically zinc alloy. Each alloy has different properties so one will waste before the other. An aluminum
Grouper is available from the manufacturer but may be a special order.
4) Complain to the marina. It is their responsibility to correct the problem.