oddly enough, the no gauss through water thing actually makes sense.
The faster a projectile, the greater a change in viscosity is going to affect it. Take a look at the mythbusters ep. where they were firing different types of weapons into a pool. Relatively slow or lower powered projectiles like pistol rounds and shotgun pellets were slowed but penetrated fairly well through the water. Rifle and other assorted high powered rounds almost instantly exploded upon hitting the water. I imagine something similar might happen with a hypersonic kinetic weapon. The shrapnel would undoubtedly scythe through the water at high speeds, but i doubt it'd pose a threat against anything armored.
