I would be tempted to look for a solution which didn't leave a heat exchanger in the path of all the crap coming down the river. You could have a settling tank off the stream, overflowing back into the stream, and have an exchanger in that or pump a flow of water from it to an exchanger above ground, returning to the river.
One of the first buildings to use heat pump technology for heating was the Royal Festival Hall on the Southbank when it opened in the early 1950's. It used a heat exchanger to take heat from the Thames. It was rather quickly abandoned due to continual failure of the river heat exchanger, but it was a new technology at the time.