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: 
### Close range combat

The monsters hide in caves in the sea bottom. When they sense the presence of a submarine they hide behind rocks and they prefer to move in narrow gorges between underwater mountain. The only way to engage them is close range combat, obviously they move faster than mechs, but since they chose narrow spaces and the submarines patrol the empty water overhead they can be encircled.

### Close range combat 2

Underwater the only available weapons are torpedoes and they are not as fast as missiles. Bullets are useless, speers short ranged. With few alternatives the possibility of close range combat becomes more likely.

### Balance

Notwithstanding all the improvements in robotics balance of bipedal machine is still delicate. Especially for a machine that should be capable of sudden changes of speed and direction and should also be able to move sideways while moving the arms for a fight. The water surronding the mech sustains it a little bit, making balance easier.
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.)