Seeing as the people of the Iron Islands are drawn from traditional barbarian, marauding civilisations, specifically (it seems to me) the Scandinavians, I'd look at what temples they used and base your initial build off of that.
So, start with something like this, the Temple at Uppsala:
Then, begin to augment it to match the depiction of the Iron Islands that you wish it to match - whether that means basing it off GRR Martin's descriptions, or off the TV Show's version of the Iron Islands. Make sure to make it similar to other places of importance, like the castle at Pyke.
Finally, add in some iconography. The Drowned God seems to me to be based not in part off Cthulhu, but don't take that as a word. Maybe work in depictions of a God similar to say, Poseidon, or other Gods connected with the sea. Where other temples have plants or ivy climbings, have seaweed. Have driftwood altars - or, better yet, have it be a driftwood themed temple, perhaps made out of the hulls of wrecked ships?
Just some ideas.
