[quote=@Skepic]
Yeah, so.... I guess [@Serpentine88]'s nation and mine are literally destined to be enemies. I mean, god damn. Their religion practically vilifies everything the Valkyerians stand for. I mean, sure, their are plenty of nations that are automatically despised in the eyes of Avalia, but this Sacred Empire is literally the anti-Avalia. Like, my god... there will be war the moment one of us even sneezes at each other.

Admittely, it will be the most awkward, go nowhere war, as neither of us could really touch each other... but dayum son, this definitely stirred things up. 
[/quote][url]https://en.wikipedia.org/wiki/APS_underwater_rifle[/url]
It was made in the 70s but technically most of the elements are already there.
Also yeah, it's safe to say that any warship in this setting would sport underbelly APS ball turrets to look out for fishmen attempting to plant bombs.

As for war between you and the Mesulids. They do have an air force and land posessions, even if not that much.
You can also try using modified torpedoes for bombing runs. Imagine a torpedo designed to travel downwards. Granted accuracy would be understandably poor in general since you can't even see your target.