To me, to "swamp" a swimmer would mean to somehow "bury" him in water. In other words, the water washes over the swimmer, either in the form of a wave or a flood. At least that's my impression of swamped.
A swimmer could be overwhelmed, though, without being swamped. An undertow, for example, could tire a swimmer out to the point that he sank under the water and drowned without actually being swamped by a wave.