To clear things up, here's the definition from the USCG on a fishing vessel. No regular boat fishing really fits the definition. Doesn't keep the people fishing from trying to claim it, though.

From Rule 3, General Definitions, Section D:

"The term "vessel engaged in fishing" means any vessel fishing with nets, lines, trawls, or other fishing apparatus which restrict maneuverability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restrict maneuverability."