It looked like the booster exploded when it submerged after soft splashdown. There was some fire and the stream cut off. Maybe that's what happened to the ship too.

IIRC the intent was to sink it using explosives (Flight Termination System) in case it stays afloat after landing.

I think they had a tug go out to drag it to deeper water and make sure it sunk - there was some ship (ocean) commentry I saw in a couple of places showing the tracking data.