More of a submersible than a submarine, since they don't cruise under cloak, and really only in the TOS era -- "Balance of Terror" has Enterprise and the cloake

More of a submersible than a submarine, since they don't cruise under cloak, and really only in the TOS era -- "Balance of Terror" has Enterprise and the cloaked Romulan ship in almost exactly the relative positions you'd expect a WWII heavy cruiser and submersible to occupy. (The Klingons and Romulans share cloaking technology; IIRC, the former are said to have bought it from the Romulans in exchange for modern ships, which is also why both Romulans and Klingons are shown to use the "Bird of Prey" spaceframe.) By the TNG/DS9 era, they've improved on the technology so that ships are shown to cruise at warp under cloak, and cloak-capable ships are shown to hold their own, without needing to cloak, against their cloakless opposite numbers. As such, they're not really any longer in the submarine/submersible role (since a surfaced submarine is easy meat for anything with naval guns); if anything, they're more like a (nonexistent) submersible cruiser, in that they can conceal themselves without needing to compromise other capabilities to achieve it.