Kayaking down the Rio Serrano is a wonderful experience and for the lucky few a fitting finale to a few days of hiking.
Some people choose to enter the national park via the Rio Serrano, taking a Zodiac boat down the river. One example is this 14 day trek in Torres del Paine.
Also, the Serrano River is a great area for fly fishing, in the shadow of the Paine Massif. The most common fish here is the Brown trout, but you can also find some Rainbow trout and the huge Chinook Salmon, all of which can be fished by wading into the river.
Photos of Rio Serrano
Map of Rio Serrano