[PlasmaKrab]

Then what about mounting such a gun on a PBR or something? If you want high-tech combat-capable coastal boats, just look towards Scandinavia. F.e. I think there should be Swedish boats around with AMOS automortar turrets any time soon, if there aren't already.
And regaring what you said about 57mm rockets on missile boats, it looks like several lower-tier forces with large Soviet stocks have the habit of welding BM-21 122mm rocket racks on old missile boats once the missile packs have become useless.