I think I'd feel safer if my ship had cannons.
Why not learn from the C-130:
AC-130H SOF Gunship, 1 105mm howitzer, 1 40mm cannon, and 2 20mm cannon. Equipped with interial guidance and GPS navigation equipment, low-light-level TV.
Quad 50s fore and aft should do quite nicely against most of today's "pirate" threats.