[hindsightbias]

What makes you think it sank? If the hull is intact it might be floating. Given the flap damage, it's probably leaking though.

[Andrew_nenakhov]

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.

[inglor_cz]

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

[stoobs]

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.