When it comes to GTA 5 vehicle mods, usually you'll encounter your standard run of the mill real-world car mods that replace the fantasy vehicles of the game with something more realistic. Of course, occasionally you can come across the wilder vehicle mod, or maybe even one that allows you to build your own custom vehicles, but these and any mod other than real cars are quite rare.
The sea surrounding the landmass of Los Santos and Blaine County is surprisingly detailed for a region that plays no real role in the story, nor has any gameplay significance. The seabed is a wonderful landscape just asking to be explored - which you can do using the submarine already present in the game.
However, some players think one sub-aquatic vehicle isn't enough for GTA V. Dilapidated and SkylineGTRFreak have put together an imposing and impressive military submarine model for GTAV and tossed another patrol boat into the mod as a bonus.
The large military sub is a truly intimidating sight to behold and is perfect for any avid Rockstar Editor fan video creators. It's size makes it less maneuverable than the Kraken, so it may not be the best choice for some deep-sea exploration, however if you're looking to do some Hunt for the Red October role-play, you'll find no better mod.
The small patrol boat is modeled after short-range coast guard boats, armed with a machine gun fixed to the nose - which unfortunately does not work at present, but the authors plan to fix this in coming updates.
While, as vehicles, these do not add a whole lot to the game, however using them for GTA V fan videos featuring naval battles or underwater scenes the mod is perfect.