The story
Uncertain: Possibly, swift + boat, or possibly based on an acronym for Shallow Water Inshore Fast Tactical Craft. The use as a political term derives from a smear campaign against John Kerry when Texas tycoon Robert J. Perry hired several Navy swiftboat veterans to form the group Swift Boat Veterans for Truth, which questioned the legitimacy of Kerry's status as a war hero.
Etymology from Wiktionary, CC BY-SA 4.0.