The more I think about this using the scenario where the death was unplanned, that's exactly what he would have done. He would want to dispose of that body and fast. He would not want to be driving around with a dead body in the car. He would look at the water and get her in there ASAP before someone notices her dead. Then he would have swum her out there as FAR as he possibly could, to where that body would not float back. He would have had a hell of a time swimming back. He would have been in a mental frenzy on what to do, and his actions were scatter-brain with the being caught on video at 6 pm yet not making it to the other side of the building until 20 minutes later. He may have been unsure what to say, when to say it, etc. He probably thought about just pretending she ran away, or took off with another friend. But his safest bet in his mind was to say she drowned and drifted off there. That way, even if the body does show up, it aligns with his story.
ETA: Also, the snorkeling story would explain to anyone what he was doing out there with her in case he was seen. He would have had her face down like she was snorkeling.