Wasn't buoyant

• Went under, like the Titanic
• Dropped to the bottom of the lake
• Scuttled
• Slumped
• Went to the bottom of the ocean
• Did well at Battleship
• Pocketed, as a billiard ball
• "Sighted sub, ___ same."

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sink \Sink\ (s[i^ ng]k), v. i. [imp. Sunk (s[u^ ng]k), or ( Sank (s[a^ ng]k)); p. p. Sunk (obs. Sunken , -- now used as adj.); p. pr. & vb. n. Sinking .] [OE. sinken, AS. sincan; akin to D. zinken, OS. sincan, G. sinken, Icel. s["o]kkva, Dan. synke, Sw. ...

Wiktionary Word definitions in Wiktionary
vb. (en-simple past of: sink )