All of APCs ingame were designed with amphibious capability in mind. Their boat shaped hull reflects that characteristic.

The Greens(USA & Italy) apc is based on the Iveco Superav

The Greys(Germany) is based on the Spähpanzer Luchs

The Browns(Soviets) is based on the BTR-90

All 3 have amphibious capability. why is it not implemented in the game?

The APCs are not based on the APCs you mentioned

For the SIK-AP APC (green) I used the Sisu Pasi APC as reference (as a tribute to Pasi, the other part of the dev team ).
The APC-GT (grey) is freestyle but the BTX APC (brown) is indeed quite similar to a BTR-90 which has been used as reference model.

We didn't make them amphibious as a design choice. This would make them even stronger and we'd have to connect the land navigation logic with the water navigation, which would be out of scope.