It had the first below-waterline flushing toilet on a boat. I've read that failure to operate the levers in the right order caused more than one of the ship's officers to take a ride on a column of water! You're also right that the same basic design continued to be used on submarines until sometime during WW II.

The "Monitor" was designed and built by Swedish-born inventor John Ericsson...and the toilet wasn't the only one of his unique innovations. It had the first revolving turret...it rotated 360 degrees, making the Monitor the first gunship to be able to fire at enemy vessels in any direction. It was also the first warship to have a 4 blade propeller. It had only 18"of freeboard, which prevented it from being a good target but also required a whole new innovative ventilation system so that her crew and officers could breathe below decks. Unfortunately the low freeboard also made her highly vulnerable to being swamped...which led to her demise and that of 16 of her crew in a storm off Cape Hatteras in December of 1862. In 1973, the wreck was discovered sixteen miles southeast of Cape Hatteras. Two years later it was designated a national marine sanctuary. Some artifacts, such as the ship's propeller and her turret have been removed from the wreck and taken to the Mariner's Museum in Newport News, VA for preservation and display.

That's just the bare bones of the story of Monitor...researching it makes for a fascinating read for those who are interested in seagoing history. The innovative designs of a civil engineer turned shipbuilder named Isambard Kingdom Brunel is another story worth spending a snowy winter day or two on.