Even moreso than tanks, submarines are pretty under-represented when it comes to videogames. The only really well-known sub game I can think of is the classic arcade shooter In The Hunt, which, coincidentally, was created by Irem, in their fondly-remembered "dirty-looking pixelart" period, while Sub Rebellion was also made by Irem in their later, more tragic "weird experimental PS2 games" period. (Tragic because it was the last stage of their existence as a games company, pretty much.)
So, Sub Rebellion, then. It's set in a post-apocalyptic future (like almost every Irem game, come to think of it), in which the world has flooded, and humanity survives in underwater cities, most of which are ruled with an iron fist by an evil empire. You play as a submarine-piloting mercenary tasked with helping the resistance save humankind, and if you have time, also finding lost artifacts of an earlier age, which can be used to develop new technology to upgrade your sub. Is it a requisite that every underwater PS2 game has to have you looking for sunken treasure?