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: 
Have you noticed that sea animals get a lot bigger than terrestrial animals? Buoyancy makes it possible to grow a lot bigger underwater due to the lesser need to support your own weight. For more details, see the "square-cube law," the effects of which are mitigated by being underwater.

That's been mentioned by a couple people already though. The thing I wanted to add is that you don't need to frame the question as "why don't mechs work on land." You can also frame it as "why are there better options on land." Projectiles, for instance, go further faster through air than ocean water, especially deep down where water pressure is high. This is true for energy weapons as well. Maybe terrestrial arsenals have a large and futuristic array of machine/rail/Gatling/laser/etc. guns that perform very well above ground but poorly underwater.

Air support is also a thing for armies. Perhaps individual soldiers can call in bombing raids, missile strikes, and so on, but underwater you've got to carry that stuff with you. So now you need the mech just to hold all of your high explosives.

So, in summary:

1. It's easier to build large structurally sound mechs for underwater use than it is for terrestrial use.
2. Projectile weapons perform well above water, but poorly underwater.
3. Terrestrial armies can call in air support, but undersea fighters have to carry all their firepower with them.

Edit: One other thing just occurred to me. You have to be in a suit deep underwater anyway. Since it's obligatory, it may as well be useful. Hopefully some combination of those reasons will suffice.
### 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.