Assuming that the naval army doesn't need to play in the standard GT scenario I think "counts as" AMTL is the best way to go.
That way you are working with units that are tested (if not 100% completely finalized) and have points costs based on that testing.
Obvisouly the restriction on "water only" movement would make such a force not suitable for the normal objective system.
I would also suggest that "coastal defences" could also be done the same way.
Heavy Defence Battery = Warlord
Medium Defence Battery = Reaver
Light defence Battery = Warhound
Again you have model for DC, shields, weapons etc. Probably reduce the points cost to account for zero move, but since the naval units can't move onto the objectives that might not be necessary.
Add some naval tranport units, some bunkers and trenches, and attacking and defending troops and tanks; and you could create a decent D-Day style scenario. Various objectives placed along the coastline to be captured and you can even keep a number of victory conditions.
DTF = defenders still hold enough objectives
TSNP = no attacking units left on the land, or beyond the beach or so.
T&H = attacker holds enough beach objectives
Blitz = attacker holds a slightly in-land objective
BTS can remain the same
If you do play such a game.. you must let us know how it goes
