Monean research vessel

From Memory Alpha, the free Star Trek reference

A Monean research vessel was a type of spacecraft used by the Moneans during the 24th century. It was armed and capable of travel through both space and water, to depths up to a hundred kilometers. They posed no threat to an Intrepid-class starship and could be easily destroyed. It is unknown if they were warp capable.

The USS Voyager encountered at least three of these vessels while in orbit of "The Waters" in 2375. (VOY: "Thirty Days")