The Collaborative International Dictionary of English v.0.48:
Gunboat \Gun"boat`\, n. 1. (Nav.) A vessel of light draught, carrying one or more guns, used for operations in shallow waters. [1913 Webster] 2. (Nav.) Any small naval vessel carrying mounted guns. [PJC]WordNet (r) 3.0 (2006):
gunboat n 1: a small shallow-draft boat carrying mounted guns; used by costal patrols