Steel Diver: Sub Wars is free to download on 3DS eshop, and free to play, in both single player as well as multiplayer local and internet modes.
If you find you enjoy the game enough, there is the option to get Premium version if you want additional content (missions, more subs).
There are also, yet further additional premium subs already out.
The gameplay in SD:SW is from a first person perspective, so this is a very different game than the old Steel Diver in 2011 that came out at 3DS' launch.
The framerate is solid and smooth (seems to be 30fps) even with 3D on.
Online, you play in teams, there seems to be all sorts of different ways to beat the other side. Potentially a great deal of replay value, which is only increased by leveling up and adding crew members to strengthen the different abilities of your different subs.
There's even a morse code chatroom before each match starts, plus morse code can also be used in-game.
Different submarine maneuvering tactics are all-important.
Single player has training modes and missions.
Steel Diver: Sub Wars represents a new business model for Nintendo (F2P). You can download and play the game both online and offline for free, without having to pay a dime for it. Yet, the Premium version for $9.99 offers more for those who want it.
From what I have played so far there is a great deal of depth to this game, no pun intended.
What you get if you pay vs what you get for free:
There's currently a further five additional historical DLC subs for 99 cents each. So about 16 bucks gets you everything in Steel Diver: Sub Wars.
No idea if there's yet more content and/or subs planned for later.
Trailer: http://www.youtube.com/watch?v=GjKYoI3ehxQ
Game Website: http://steeldiver.nintendo.com/
Developer Website: http://www.vitei.com/
Price: Only available on 3DS eShop, Free to Play or $9.99 for Premium version which is the full game, and 5 more subs available within the game's own shop.