The AAV-7 that Katsuragi is holding is amphibious, but it's kind of a balancing act to make something that has some resistance to current anti-armor weapons and also floats in the water. If you watch videos of AAVs launching from an amphibious warfare ship, they drop off the stern ramp and (hopefully) bob back up to the surface! If the ship is trimmed so that the stern ramp is a few feet under the surface, it makes the drop a little less severe.
AAVs are capable of driving a considerable distance in the water, but the ride is reportedly rather unpleasant. They supposedly can go faster than the Type 2 Ka-Mi though! On the other hand, the armament (.50 MG and 40mm GMG) leaves something to be desired if you are going up against anything harder than light vehicles.
