For clarification, the way I see the Atlanteans in this DC Universe we've crafted varies between the two kingdoms. The grunt soldiers for Poseidonis look like the Atlantis grunts in the animated Flashpoint Paradox film. 

[img]https://media.discordapp.net/attachments/405206901534752779/462569887651463168/1000.png?width=400&height=226[/img]

While the armies of Xebel rely more on water constructs and more traditional looking Aquaman styled armors.

[img]https://vignette.wikia.nocookie.net/dcmovies/images/9/92/Aqualad_Garth.png/revision/latest?cb=20150130220729[/img]

Here's a quick scene of all three in action against a Navy sized war ship. The grunts, the water benders of Xebel [Aqualad I and II] as well as the mystic stuff with Orm/Ocean Master. Attitude wise Orm would be more like the Flashpoint Aquaman here.

[youtube]https://www.youtube.com/watch?v=hhU47qhpGrE[/youtube]

It should be noted their armies are great in number and their tech and mystic advantages are numerous. In the clip the Navy even had metahumans on it's side and were still not match. Take into account we'll also be dealing with giant kaiju sized crab monsters and sea creatures this is as big as you guys wanna make it. 

- Ω