Question: In my world, the planet (which is not Earth, but is suitably Earth-like) has been besieged by deadly monsters for about 1000 years. Their initial attack lead to an apocalyptic scenario which took humanity hundreds of years to fully recover from. Since then, humanity has banded into one large empire/federation and another apocalypse has been averted, but it requires periodic military excursions into the wild to prevent any of these monsters from entering city limits.

The most dangerous variants of these monsters are the ones that dwell in the sea. To combat them, part mechanical part biological mechs were built. They are 15 ft tall and fight alongside small and medium-sized submarines. They are very effective in their role, but are not used in land-based operations. Instead, soldiers either fight with something like the exoskeleton from the movie Elysium (Here: ![Text](https://img.washingtonpost.com/rf/image_1484w/2010-2019/WashingtonPost/2013/08/22/Health-Environment-Science/Images/hs-elysium0827.jpg?uuid=G6vcQAtoEeOJdPl6s7PGdw) or just have simple ballistic vests that protect them from possible friendly fire.

My question is: why would these mechs be limited to water combat if they have been so effective in fighting monsters?

Answer: 
Another possible reason would be fuel. A giant mech is going to require a metric crap-ton of energy to move around. Perhaps the biological components of the mechs are developed from the monsters themselves, and generate large amounts of electricity from seawater. (I don't know the science, but I'm sure there's something involving high-electrolyte water or trace metals suspended in saltwater that will suffice. Or maybe they filter trace radioactive elements from the water and use them for micro-fusion. Or...)

Thus, a mech *can't* operate on land, because it can't carry enough seawater around to operate for more than a couple of minutes. But when immersed *in the sea*, there's no such thing as running out of seawater fuel. Functionally, this is the same as an air-breathing animal needing to stay on the land -- we use oxygen specifically to generate energy, and need a continuous supply of it. The mechs use seawater to generate energy, and need a continuous supply of it.

(This could also lead to a cool story idea where the monsters discover this weakness, and start deliberately throwing/pushing the mechs above the surface of the water to weaken or disable them. Like fighting a human in a river and shoving their face underwater to take away their oxygen supply, only in reverse.)
Missiles.

In the war between weapon and armor the anti-tank missile has prevailed. Mecha are basically a tanks that trade some armor for better handling of some terrain.

However, there is no underwater equivalent to the infantry anti-tank missile. There are torpedoes but they are a lot bigger and don't move nearly as fast--your mecha have point defense systems that can stop torpedoes. (Yes, there are supercavitating torpedoes, but during the supercavitating phase they are blind. You either use them as unguided weapons or you use the supercavitating capability to get them close to the target and then drop to normal torpedo speeds to actually attack.)