There is an an underwater tour available of the great sights under the Blake Sea that you can take from Flotsam City. You pass sunken pirate ships, sea agricultural farms, towns for merman and mermaids, an It spans many sims and lasts at least 30 minutes.
Here is the SLurl to the entrance of the submarine tour:
Here is my review and more photography from Steampunk-inspired Flotsam City.
There is an underground laboratory in Flotsam City, Captain Nemo, which is very well done. You can see my review and more photography here: