[andrewjshults]

Having swam in the Hudson (NYC Triathlon) and kayaked extensively in the East River (including a trip up the Gowanus Canal - never again), the water quality in the main parts of the river is actually quite good these days. The biggest thing to keep in mind is that most of NYC runs the gutters through the sewage system, so when there is heavy rainfall, the combined output overwhelms the capacity of the system and they release untreated sewage into the rivers. There's a lot of community involvement in tracking water quality in the rivers and you can get real measurements at a fairly regular basis: http://www.nycwatertrail.org/water_quality.html I'd be far more worried about being an unsupported swimmer. Even in a kayak, the danger of boats is quite real and swimmers are even less visible (and slower) than kayakers.