well as i have posted before.
it cant be a merchant ship... it would have to be a sleek, large motor yacht.
this way, if the situation goes too far south, it can turn tail and run at high speed.
second... it appears that most of these craft are being approached by small boats... so you dont want to be armed with a typical cannon... but several of these would work nicely at cutting down any threat.
such a weapon could be fairly easily concealed aboard a yacht.
for larger long range defense, the 5 inch gun i think... on some sort of hydraulic lift or concealed in some sort of container with fold down sides.