It's a shame that so many people are going to overlook this underrated gem
While the pacing appears slow, it's actually a very involving game that will keep you on your toes. Each aspect of the sub is operated with a different control, so you have one slider for horizontal movement, one for vertical, one to control the pitch of your sub, separate firing controls for each torpedo bank, and a "masker" for throwing homing torpedoes off your scent. While your sub is moving sedately across the top screen, you're busy on the bottom screen keeping it on course. There aren't too many moments when you're not tweaking one control or another and so the measured pacing is actually a necessary part of the game design to keep it from becoming overwhelming and unplayable.
Steel Diver is fundamentally an arcade game, but there is a simulation element to it as well in that the sub doesn't respond instantly to your input in accordance with the laws of physics, specifically inertia. That means that throwing your sub into full reverse just before hitting an obstacle generally won't work since your sub will continue forward for a few feet before slowing down and reversing course. This is where most of the challenge comes from since you have to think a few seconds ahead and plan your moves accordingly. Thankfully there's a map on the bottom screen that can help you out, though it only shows basic terrain. You'll have to pay attention to the top screen to spot enemies, mines, and other underwater hazards. It can get tricky at times, especially when you're trying to line up a shot on an enemy while avoiding their torpedoes and other obstacles, but the game is manageable for the most part and shouldn't pose an insurmountable challenge to anybody.
Initially there are only 5 mission levels available with 2 more that unlock once you've beaten the first 5 with all three subs. This isn't as monotonous as it sounds since each sub has unique characteristics that will change the way you approach each mission. The Manatee is the smallest and most maneuverable of the three but is also the most delicate. It's the only sub without a pitch control, but it does have a handy top-firing torpedo in addition to its single front-firing torpedo. The BlueShark is mid-sized but is still reasonably maneuverable with two front-firing torpedoes. And finally the Serpent is the largest and fastest sub but also the least maneuverable. However, it's the toughest of the lot and features four front-firing torpedoes, so it can really lay down the hurt on the bad guys. It's definitely worth it to replay a mission with a different sub because it offers a new experience.
There's a nice variety to each mission level, so you don't feel like you're playing the same thing over and over again. They all have a distinctly different look, too, which makes each one a visual treat. It might not look all that spectacular in screenshots, but the 3D really makes things pop. There is a wonderful sense of separation between foreground and background elements, and it's like you're looking through a window at a miniature underwater diorama. It sounds like a small thing, but it really adds to the atmosphere of the game. Sound effects and music similarly add to the overall sense of immersion and do a good job of conveying the feeling that you're underwater. The subtle sound of water moving across your sub's hull is probably my favorite.
But you might still be thinking that 7 missions just isn't quite enough. Perhaps one of Steel Diver's best kept secrets is that once you've beaten all 7 levels with all three subs (which took me a solid 5-hours or so), a more challenging Expert Mode opens up that reinvents each level with new obstacles and enemies. And just so you know, Nintendo didn't call this "expert mode" for nothing. You're going to have your hands full keeping your sub in one piece.
After finishing each mission, you're taken to Periscope Mode which utilizes the 3DS's gyro sensors to simulate looking through a real periscope. You hold the 3DS up and spin around in place to rotate your view across the horizon to locate and sink enemy ships or submarines. Each enemy vessel sunk earns you a random decal, and if you collect enough of each kind you can put them on your sub for different perks like extending the mission time or reducing the amount of damage you take (decals don't stack, in case you're wondering; you can only use one at a time). And don't worry, if you're in a situation where you can't spin in circles with a game system held in front of your face, you can use the compass on the touchscreen to rotate the view, though this isn't nearly as quick or precise as using the gyro sensor.
There really is a lot of content here, and I haven't even touched on time trails (either mission levels or 8 original time trial levels) or the Steel Commander strategy game which can be played solo or with a friend through "download and play".
Please don't listen to all the reviews selling Steel Diver short. It's an engaging and fun game that will keep you thoroughly entertained for hours and is well worth the asking price.