This sounds a lot like
Surfboard, a unit which used to exist in Zero-K and was removed. Surfboard was probably more tough and more costly than what you are envisioning here.
Surfboard saw almost zero use for a few reasons:
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Since it carries a meaningful expense, shipfac is (deliberately) a bit overtuned, and ground units would barely be able to interact with underwater amph, most ground units on a Surfboard are uncompetitive in combat on sea.
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Despite that, it would have been scary to buff Surfboard due to a few units that would abuse its mobility (think of Moderator, for instance).
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In the "bring a land army to the other side of a body of water, without exposing it to AA fire" role it is wildly outperformed by Djinn. A single Djinn can handle quite large armies fairly quickly, doesn't have to expose itself to surface fire, and doesn't have to expose its payload until it is ready to deliver it.
It might have hung around in ZK despite not being very useful, but the other problem with Surfboard was that a unit being able to fight while in a transport is a unique and special case. Any time a change was made to the mechanics of the game, one always had to ask the question "but what about Surfboard?" to avoid introducing unintended behaviour. Eventually the development cost of maintaining a unit which saw next to no use became too great to justify.