The Xindi-Aquatic cruiser was a type of starship used by the Xindi-Aquatics during the 22nd century. The vessel was at least 1,800 meters long, heavily armed, and equipped with the most advanced technology of any of the five Xindi species. The ship featured a hydrodynamic, swept-wing hull structure and a large hangar bay in the primary section that could be used to carry other starships. Despite also having superior weaponry, the Xindi-Aquatic ships had a drawback of being slower than other Xindi ships.
One of these cruisers participated in the battle to stop the Xindi weapon, after it was commandeered by the Xindi-Reptilians, and transported Enterprise NX-01 into the battle. It was disabled during the battle by a spatial anomaly generated by a nearby Sphere. (ENT: "Countdown") Another cruiser transported Enterprise back to Earth following the successful destruction of the weapon, probably to shorten the trip as a thanks for destroying the Spheres and Sphere-Builders. (ENT: "Zero Hour")
These Xindi-Aquatic ships used a small room where oxygen-breathing humanoids could come aboard and communicate with the Aquatic crew. A round window was located between this room and the Aquatics' domain. (ENT: "Damage", "Countdown")
In the script for "Countdown", the Xindi-Aquatic cruiser was introduced with the description, "Five times the size of Enterprise, the Aquatic ship is an impressive sight."
The round window aboard the cruiser was previously included in an alien bazaar in the third season Star Trek: Enterprise episode "Rajiin" and went on to appear as part of a Romulan control room in season four's "Babel One" and "United". Interior views involving the Aquatics' section of the cruiser were rendered with CGI, even to the extent that the water containing the Aquatics themselves was digitally created. Although a detailed control room (including control consoles) was designed for the Aquatics, the murkiness of the artificial water mostly obscures this part of the vessel class. ("Countdown" text commentary, ENT Season 3 DVD)
Executive Producer Brannon Braga liked the design of the Xindi-Aquatic cruiser. "It's not every day that you can go on a ship filled with water," he observed. "Only in science fiction could you have your captain standing in what we call our 'airquarium.' I've never seen anything like that, and it was fun and cool." (Star Trek Magazine issue 117, p. 62)
In Star Trek Online, this ship is called "Xindi-Aquatic Narcine Dreadnought Carrier", and a playable version is available by opening Xindi Lock Boxes. As a carrier, the ship deploys small craft similar to the Xindi-Aquatic scout ship (called "Mobulai Frigates") for use in combat.