See the information sign half way down the pier for a history of the various ferry piers built in Norway Bay, going back to 1850. The current pier was built in 1913 and is built of good ferrous (magnet-loving) steel. (Did you see how I just slid that hint in there?)
This cache has been muggled fairly regularly... you're looking for a hide-a-key container. Warning that this is a very busy spot in summer, so please watch for muggles and be discrete if searching in July/August.
Lastly, please note that this cache can be accessed from on top of the pier. No need to get wet or bring a boat!