I had the unique pleasure of being presented this delightful game recently: a fully working, mod-free "Battleship" game, right inside Minecraft, by magib1! Battleship has come in many forms over the years, most commonly as a board and/or computer game, where players deployed their ships across a grid, then took turns trying to guess where their opponent's ships were, and sinking them.
magib's map creates the game in more ways than one; aside from having the ability to place your ships as you see fit, and the usual "take shots at where you think their ships are" mechanics which are standard, his map also spawns a ship model for every ship a player deploys, which will get "shot" with deployed TNT on a hit. It's crazy fun!
If you want to try out this insane map for yourself, be sure to drop by the Battleship thread, which has the download links. Enjoy!