Based in Potrerillos rather than in the city, which matters more than it sounds: you can drive out, park, and be on the water without the hotel pick-up and the hour on a minibus. They run the Río Mendoza rafting, kayaking on the Potrerillos reservoir, and quad-bike trips into the hills above it.
The reservoir itself is the draw for anyone who does not want white water — a turquoise lake with the Cordón del Plata behind it, and kayaking on it is closer to a sightseeing trip than a sport.
The coordinates come from the map on their own site. They take bookings on WhatsApp.