Right, "Meta has went out of ideas, how can a submarine be useful in a battle against floating bloons", you think? Don't worry, I've managed to work it out!
The Monkey Submarine has 2 modes (shown as 2 new attack modes buttons): Submerge and Surface. While it's Submerging, projectiles can't hit it unless it's a lightning or electronic-based projectile (since seawater is very conductive). It has 2 cannons, one shoots homing darts, these darts can pierce 3 bloons each. This cannon can be used on both modes, since the homing darts will fly upward, out of the water first if it's Submerging. The second cannon however, is a frontal cannon, and acts just like a Monkey Buccaneer's cannon. This cannon can only be used while it's on Surface (if the submarine is on Surface, BOTH cannons can be used!). The 2 cannons have the same fire rate. Its range is a bit smaller than a Monkey Buccaneer, but bigger than a normal monkey.
Price: 950$ on Medium
The submarine now has 2 Frontal Cannons side by side.
Price: 500$ on Medium
Advanced Heating System
The homing darts now become missiles that explodes in a small radius and can pop Lead Bloons. The Frontal Cannons' darts have also been heated up to pop Lead Bloons and frozen bloons, and the overall fire rate has been improved.
Price: 1100$ on Medium
Adds a Wave Cannon to the submarine's arsenal. This cannon can be used on both modes, though it will do different things. When Submerging, the cannon will emit a sonic boom in the water around it, stunning all the bloons in its range if they're on water, but do no damage. When on Surface, It'll compress water energy and shoots an aqua-energy crescent-shaped "wave" that's as wide as Monkey Buccaneer's beam. This wave has unlimited pierce, and will pop all the bloons it comes by by 2 layers while freezing them.
Price: 3200$ on Medium
The NS-32X "Nautilus" is a renowned nuclear submarine, so why don't we nuke things up with it? This submarine has its homing missiles launcher reinforced with nuke, so the missiles will now explode in a MUCH larger radius while popping a whopping 3 layers while burning them! Its Frontal Cannons have now also be replaced by THREE nuke-powered Phase Cannons! These Phase Cannons have unlimited pierce, can pop 2 layers off each bloon while stunning them from the shock! The Wave Cannon's waves are now also twice wider!
Price: 12800$ on Medium
The submarine can now detect camo bloons, and its range has also been improved a bit.
Price: 350$ on Medium
When you turn back to Surface mode after it has been Submerging long enough, the submarine now has a small chance to find something valuable underwater and pull it up for you to use! That "something valuable" is either a small treasure chest worth 100$-500$, or a common loot item.
Price: 1200$ on Medium
Sure, aquatic torpedoes will be no good against floating bloons, but Glider Torpedoes are a different thing! Now adds a Glider Torpedo Cannon on the sub's arsenal. This cannon will shoot torpedoes that, when meets land, will fly STRAIGHT upwards after the collision and hits the bloon right above it, if there's one. It'll then explodes in a medium radius and pops the bloons by 3 layers or 15 HP. However, if there's no bloon over it when it flies upward, the torpedo will be completely useless. That's when Submerging though. On Surface, however, the torpedoes will just shoot straight, resulting in much better accuracy.
Price: 2200$ on Medium
The JV-61 "Javelin" is weird! It's shaped somewhat like a mechanical swordfish, with a long thin drill on its head. Of course, such design gave it an unique ability...
Special Ability: First, you'll need to choose a destination point on the track. It'll then use its engines to create a massive water blast that allows it to "jump" to the sky at extreme speed while using its long, thin drill to deal a whopping 1000 HP damage to every blimps and bloons it comes across! When reaching its destination, it'll land "on" the ground by digging a vertical hole with its drill, then drill all the way back to its old location. That'll create a hole on the ground where it landed, though it's only for visual effects since the bloons are floating so they're unaffected.
Because of this horrifying and unexpected move, it's also codenamed "Leviathan" by the bloons!
Price: 8000$ on Medium