The US used a concrete ship (YO-160) [1] as one of the target/target vessels during the Operation Crossroads tests [2]. The ship had been used as an oiler/fuel barge during WWII and sunk not from the tests directly but from damage sustained when moving it after the Able test. Although the Baker shot served as a coup de grâce (ultimately sinking the vessel), YO-160 was already taking on water and probably would have sunk on its own.
They are not "boats" exactly!
http://www.concreteships.org/ships/ww1/atlantus/postcard-bre...
Really is quite amazing that you can float a 600,000 tonne object several hundred miles out to sea.
[0]: http://movingimage.nls.uk/film/5973
[1]: https://en.wikipedia.org/wiki/Loch_Kishorn#Kishorn_Yard
https://www.washingtonpost.com/news/morning-mix/wp/2017/01/2...
For more concrete fun, and interesting material sciences challenges, see what floats civil engineering students' boats: http://www.asce.org/event/2017/concrete-canoe/
http://www.worldnavalships.com/forums/archive/index.php/t-14...
Crappy ships, but they are fantastic artificial reefs. We used to kayak or boat right next to them and catch tons of fish. The area is now a state park.