Instead of buffing only stats of underused existing units, why not take advantage of real-life amphibious properties of the armored units?
I.e. the BTR, BMP, BRDM could be able to cross water surfaces in AI bases at reduced speed.
Or, on the map, let these units be able to travel on the first water hex off the coast line at reduced speed. A maximum of 2 hexes off the coast could be implemented to avoid defenders, although doing this would damage the units gradually until destruction. This will prevent campers. Maybe include helicopters with that.