Here’s a lovely selection of marine-themed door knockers. There are some repeat designs and some that must surely be unique
These two are the same design and by coincidence both are not in seaside locations
Fish door knocker in Woodstock, Oxfordshire
A chunky trident-tailed example
A crab, adorning a door in a seaside town famous for crab
Rather stylised fish on this island door
Fish door knocker in Portsoy, Scotland
Which came first, the spyglass knocker or the name of the house(also spyglass, if you hadn’t guessed)?
On the door next to the Nelson pub
It looks like this could be a double knock, tail the head. Of course I couldn’t have tried it out people think I am strange enough taking photos, imagine if I started knocking them as well
A bit blurry, sorry. The pavement was very narrow and there were many tourists
Maybe the captain lives here?
their safe port
another landlocked example.
Knockers and Knobs 
I love the wheel and the crab ones!