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: 
**They are too heavy to go on land.**

These mechs have seriously thick armor. The weight of this metal is balanced by gas filled spaces. The underwater mechs are neutrally buoyant. Because of their mass they have inertia but they do not need to constantly fight the acceleration of gravity. They can walk along the bottom of the ocean fine but their servos would not be able to carry them on land.
**First, for the biological part**

We are starting with (presumably) as our base a subspecies of this monster which has adapted to marine life. Over this short a period that is pretty remarkable and suggests a very fast rate of mutation. Adaptations can include things like:

* streamlined hydrodynamic shape to improve speed underwater. For this reason I suggest the mechs be 15' feet *long* not *tall*. This will cause a tradeoff on land. Just look at Crocodiles: they move fast as lightning in the water but are very slow and ineffective predators on land.
* a shift from limbs to fins or flippers and tail flukes
* either develop the capacity to 1- hold breath for a very long time like whales or 2- evolve gills to to get oxygen from the water.

Option 2 would explain why this specific model of bio-mech will not work on land. The living part of it wouldn't be able to breathe. Option 2 would also allow this creature to dive to depths well beyond what a human diver even in a suit can survive.

**Now, as for the mechanical part**

Submarines are good at hunting other submarines, but marine life is better at detecting other marine predators and prey. Electrosense, the ability to detect minute changes in water pressure which suggest the approach of a fast moving animal, keen eyesight, echolocation.. a submarine just has sonar and has to be careful how and when it uses it.

By the same token, firing a torpedo or anti-ship missile at wildlife is... well it is significant and ludicrous overkill, and a good way to run out of missiles and money. Something armed with weapons more suitable to combating an underwater predator though, and mounted on a bio-mech is a feasible way to fight a war. The submarine's role in this mode of warfare would essentially be to act as a tender / base of operations for teams of these biomechs.