My gods people.
Get it into your heads.
MEREPEOPLE = NOT AWESOME. NOT USEFUL. YES LAME
They are usually confined to the sea, and whenever they can go on land they are very weak humans
at best, usually being crippled!
There may be nations in the sea with which we may want to deal- but think about this- the land can always attack the sea (boats),
but not vice-versa.
And currently our enemies are on land.
Our enemies sea-based operations are the following:
With an army of merepeople, we could indeed- easily- prevent this from going on. Woop woop. Apparantely the land transport is good enough it takes a few days to raise an interspecies army- they can deal with no sea trade for a while.
An army of merepeople might- might- be suitable for attacking coastal cities. It would be very difficult- even if the enemy has no walls between the harbour and the main city, unless the humans are venetians, capturing the city will require going over land. Mermaids on land= not cool. In the best possible situation (barring venice), with a city with no walls, right up on the shore, we would need at least double the garrison in order to over power using mermaids. And by garrison I also mean peasants with pitchforks, because, hey, they could defeat land-bound mermaids if they try.
And then we would need a large garrison, because guess what? People don't like strange people! They will deal with them, but if you try to rule over humans with mermaids, you will get resentment!
ALSO: Mermaids are usually not barbarians.
Wereas desert cavalry- oh, desert cavalry! I could go on for hours and hours!
I am super, super cereal when I say if we pick mermaids instead of desert cavalry there had better be a very good deus ex machina why the mermaids are super awesome. And even then I shall moan endlessly about how it makes no sense.