I like your ideas so far, but I'd like to present a couple others as well. Keep in mind that I'm terrible at naming things, so I'll just present the concepts. Maybe somebody else can help out with the names if any of these ideas are used.
Based on the StarCraft Scourge unit, but remodeled for naval warfare. It would need to be relatively inexpensive (~200-250 credits), very quick (~15 mobility) and deal solid damage from close range (~5 to 7 attack from a range of 1). However, it would have a very low defense (defense ~2 or 3). It would be designed as a sacrificial unit to be used to soften up an opponent prior to Leviathan attack. A group of these could even take down a fully healthy Destroyer on their own, but the numbers should be balanced in a way that this isn't cost effective. It's not designed to be a stand alone unit, but to be used in conjunction with the rest of the Naval and Air fleet of the Khraleans.
Alternate 1 - Could be given the 'Plague' special ability to give it a weapon when it is unable to approach to a range of 1.
Alternate 2 - Could be made into an amphibious unit to be used in a similar way as the Sapien Tankbuster proposed by Sims. Land movement would need to be slower (2-3 hexes perhaps?).
This would be a submarine-type unit that could only attack other Aquatic units. It could dive under the water and resurface to attack (same mechanism as the Underling's bury ability). High damage (~8 base attack with +4 bonus on resurfacing) but low staying power (~4 defense). Moderate speed (3 hexes on surface, 2 hexes when underwater). Cost around 350-450 credits. The Underling of the sea.
A versatile Naval unit, almost like a Garuda on water. It would be moderately priced (~450-550 credits), quick (~12 mobility) and have an attack range of 1-2 hexes. Moderate damage (~7-10 attack against all unit types) with solid defense (defense ~9 or 10). This unit could be effective on it's own, or act as a front line to protect the powerful yet vulnerable Hydronaut.
Will edit with the other 3 units later.