Several Issues with that actually.
One, our base is Landlocked so we have neither a decent Deployment nor Testing Area for this.
Two, We have Precisely Zero Naval Engineers or Researchers at this time.
Three, we already have several ongoing projects that we really need to focus on.
Four, not actually an Issue per se, but various military branches of the Union are starting their own Super Robot Projects. We might as well let a Naval backed Project focus on Naval Tasks.
Narratively our big tie is with the Legion and thus fighting the Kaus. We got some links to Sheol via our K-Class, but honestly, he's still focused way more on Mercury. The Westaphalians are a concern for the Union all around, but we're more reactive to them, and our Mass Production frames are giving the Union sufficient leverage that we're keeping up with Conventionals.
If we try and diversify to put out Every Threat, we won't actually be able to specialize to the point where we can hit outside of our weight class.
We're not alone, we have friendly factions that are supporting us and being supported by us. We don't need to hold up the World, just brace enough so that our friends can help with it.
Testing: Just build a jumbo-sized aquarium. Deployment: This is why we make it
amphibious instead of
aquatic.
Lack of Naval Engineers: This is why we
reach out to the Marines to ask about them; if we never try to recruit them, how will we ever get one? (And as a side bonus, this would give us additional actions which we could then focus primarily on our naval Super Robot lineage, which is a net gain for essentially no loss because we aren't losing our
other Engineers.)
Ongoing Projects: You're acting like an amphibious robot wouldn't benefit from most of the same projects as our terrestrial ones. The ones they don't benefit from, well, we can just stick our Naval Engineers from my above counterpoint on making specialized equipment.
Leave It To Navy: I've said this before, but us building Super Robots opened the floodgate by proving it's possible. You can't prove that it won't be the same for amphibious robots; all we'd need to do is make one amphibious design to prove that Super Robots have their place in naval battles (as in, not merely "robots so big they can stand upright in water," but
actual naval Super Robots like a Configuring submarine), and that could be the impetus for the Navy to get in on this "start a mecha project" spree.
Narrativity: We are enemies of every antagonistic faction, and as such I feel we should have at least one machine capable of fighting each of them on their home turf. Otherwise we're nothing but a reactionary force, and we all know that a purely-reactionary combatant is going to be the loser eventually.
Friendly Factions: Even more of a reason to make an amphibious machine, in my eyes, because if we don't that means we won't have the means to
support them.