Hypothetically, it may have been physically possible to tow the floating first stage back to shore, but there was no good reason to do so. The SpaceX boosters are designed to be recovered and reused IF they successfully perform a soft landing on a hard surface. If, instead, the booster is dumped into salt water in an uncontrolled manner, it becomes practically useless for reflight. So at that point, the value of the booster became zero, and the cheapest thing to do was obviously to sink it. Since the US government had a legal liability in the matter, the Air Force could be officially tasked to deal with the matter.
a reply to: Zelun