Something tells me that realistically more firepower than that would be needed. My first weapon of choice would be some .50 cal machine guns mounted at strategic points on the ship and lots of ammunition but I’m not really an expert.
The US navy, which has long mounted the old M2 .50 against just such smaal boat raids as that which nearly sunk the U.S.S. Cole has rethunk their position about such things, concerned about, among other things, collateral damage.
Armament Systems Branch Fields New Gun System on Deploying Ships
CRANE, In -- Naval Surface Warfare Center, (NSWC) Crane was tasked by NAVSEA (PMS NSW) in direct support of Chief of Naval Operations' (N764) Operation Task Force Hip Pocket to install additional small arms weapons on deploying ships for force protection. The directive to provide MK44 systems to several ships scheduled for near-term deployment was received late last year and in less than 45 days, 56 systems were procured, assembled, inspected, and delivered to the Truman Battle Group (BG) ships in Norfolk and the Nimitz BG in San Diego.