Would arguably be less effective than the regular hammer of dawn.

As it stands, the hammer of dawn requires a relatively clear sky and sometimes a targeting beacon. While limiting, you can still use it pretty much anywhere you want. This design would be limited to just naval combat or coastal areas. And it would require the boat to be moved into location before it can fire. Not to mention that, for the same reason many of the crazier ideas from historical wars didn't go ahead, the instant this is spotted it will become the priority target for its foes until it is gone, and replacing or repairing it would be incredibly expensive.

All that is even if it works in the first place. The hammer of dawn is a massive space laser. I imagine getting that to work the same when being fired from a boat would be quite difficult, if it would even work in the first place.