Both are over-engineered for sure.
A modern marine corps would be an ideal choice for the airborne, swamp operations and river/lake crossing niches.
Too bad, the USMC is redundant with army formations (101st, 82nd, and army in general as demonstrated in WW2 amphibious operations in the European theatre).
It makes sense to give a marine corps an amphibious AFV, but that doesn't need to be a high seas or high speed design. A winch and good climbing ability are more important (many rivers are between rather steep hills).
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