Per the HOI4 wiki
Shore bombardment:
If surface ships are on hold in a province adjacent to a land battle involving enemy divisions, the ships will automatically perform
shore bombardment to reduce the enemy's attack and defense.
The magnitude of this effect is determined by the firepower of the ships involved: each point of heavy gun damage contributes
-0.1% to enemy attack and defense, while each point of light gun damage contributes
-0.05%. Total reduction from shore bombardment is capped at
-25% (even if you have traits that increase shore bombardment).
The General trait Naval Liaison as well as the Admiral trait Ground Pounder increase shore bombardment, but not beyond the -25% cap.