Free version available at https://play.google.com/store/apps/details?id=com.jandric.seacraftfree and http://www.amazon.com/s?field-keywords=seacraft+multiplayer.
Not so free on WP8: http://www.windowsphone.com/en-gb/store/app/seacraft/117254d4-c29d-45bf-bcb4-99bd283aa65f