May I suggest a particularly interesting phenomenon which you can find in ocean basins (and in the arctic regions)?
Brine pools are patches of extremely high salinity sea water which form on the ocean floor, typically when salt deposits dissolve or when the formation of ice causes dissolved salts to become concentrated. They are oftentimes surrounded by life which feeds on chemicals which well up from the brine (or what created the pool, anyway). These pools are distinct from the surrounding seawater, and do not mix with it due to how extreme the salinity difference is. You might be able to find shallow water brine pools in arctic regions, which would make those areas more interesting. The surrounding life might serve as a food source...if your dwarves can handle getting into arctic waters, or you get "inventive".
In the same lines, I'd like to propose making more forms of unique arctic sea life-there are some animals there, like the
icefish, giant jellyfish with tentacles so long they can drag across the ocean floor, leopard seals (powerful predatory seals that prey on penguins, fish, squid, and even other seals, and has caused a human fatality), penguins, greenland sharks (relatively primitive), colossal squid (which easily outclass the giant squid in weight if not length), and many other little-known creatures. The arctic waters are actually very diverse compared to the land ecosystem.