The buoyancy part is not hard to achieve in AFV design. The problem is the propulsion. To have some decent level of amphibianess, the usual means of propulsion is by track or by secondary prop driven system. Propelling a AFV in water using only tracks is slow and cumbersome. Propelling it using boat propellers at the rear of the hull is more efficient but more complicated due to a second motor at the rear for the props, and powering that.
Waterproofing the vehicle is a pain before water ops, but what is much worse, is the vehicle usually cannot fight right away when it crosses. The waterproofing has to be blown off or removed. Maybe under fire.