A game that can be explained as "vaguely reminiscent of Silent Service" doesn't have a large audience.
Steel Diver is a weird, unorthodox game. There's really no other way to describe it. The controls seem deliberately difficult, and the general progression through the game is peculiar. Still, it's fun and rewarding in its own way, though it is certainly not for everyone.
The controls are where a lot of the game's uniqueness comes from. You control a submarine through a variety of missions, lasting around 10 minutes each, by using dials and switches on the touch screen. You set how fast you want to go forward or backward with one dial, and then how fast you want to submerge or dive with another. You have three subs to pick from, each with slightly different abilities such as firing torpedoes upwards or fine-tuning the pitch of the ship to more accurately attack enemies. It's definitely obtuse, but it's easy and enriching to use once you get the hang of it.
The goal of each stage is to reach the end as quickly as you can. Along the way, you have to maneuver through tight underwater passages, dodge torpedoes and floating mines, and defeat enemy subs and creatures. The weird part about the main missions is that there isn't any kind of score mechanic. Instead, you're ranked by your time. There are eight additional Time Trial missions, along with Developer Ghosts to race against for each mission, and while they're fun, they don't offer much of a difference from the main levels. Also, to unlock all the game's levels and content, you have to beat each mission with every sub. While they control slightly differently, the missions are identical with each vehicle.
At the end of each mission, you enter Periscope mode, where you move the 3DS around yourself in a similar manner as for Face Raiders, but without the augmented reality aspect. Instead, you're looking through a periscope just above the water's surface scouting for ships to shoot torpedoes at and destroy. The goal in these mission end-caps is to destroy as many ships as you can in the time limit. Your rewards are decals, which after you collect a requisite number of them, are unlocked so that you can put them on your submarine. The decals have the effect of changing the game a little bit, such as negating water currents or making your torpedoes more effective. In order to even unlock decals at all, you either need to be an idiot savant at the game, or replay each mission in its entirety several times. You can play the Periscope mode outside of the missions in three different levels; however you don't earn any decals that way. It's just another high score mode.
The final mode in Steel Diver is also probably the best-kept secret about the game. Steel Commander is a Battleship-esque mode where you have to move your ships around a grid-based environment and try to destroy your enemy's supply ships without getting yours destroyed. You can't see your enemy ships unless you land on their spot, or use sonar to pinpoint their location, which reveals yours. Submarines attack by using the periscope, and your other ships attack by dropping depth charges at one of three different depths. You can defend against the depth charges by picking a different depth than your opponent. This mode is very fun, and the variety of maps and degree of strategy give it longevity. Sadly, the only way to play this mode is against the computer or via Download Play. Hot-seat or online multiplayer are sorely missing.
Graphically, the 3D presentation is wonderful. As corny as it sounds, it feels like you're looking into an aquarium, as there are multiple layers of fish and underwater foliage. It's especially cool when an enemy fires a torpedo from the back layer, seemingly coming towards you.
Steel Diver is most definitely a niche title that most people won't care for, but it's a good game in its own right, despite its brevity. The mechanics, while a little obtuse, are interesting, and the three different game modes are varied and fun. It's just that there is a large focus on replaying the same content with minor differences. If you happen to have someone around who you can play the Steel Commander mode with, you'll likely get some longevity out of this game. Aside from that, it's a quick romp with minimal rewards that is worth a look but not much else.