You guys are thinking too big. Gotta go for something that was designed to be mounted on a boat, like the Phalanx gun (or, US Navy Close In Weapons System CIWS).
Firing rate is nothing short of unholy. The guns are designed to shoot down suicide boats and even incoming missiles (with a rifle? yup, when it’s this one!). Somewhere on the web there is a video of it vaporizing a small drone boat during initial test; one about the size of those Mississippi cruisers.