Real combat is always the best teacher. The most well-conceived tactics in plans and blueprints might shatter on the battlefield with a single blow.
Once, humans and Fishmen compared their underwater maneuverability... Even with the help of underwater motorcycles, even with the boost of Divine Blessing in this regard, humans spent a tremendous amount of money to gain the upper hand in speed, only to find it meaningless.
What use is sheer speed? Your underwater motorcycle follows a straight path, but can it change direction like a live fish, accelerating and decelerating along the underwater currents?
Not to mention the Fishmen's standard "Radar Skin" and "Sonar Exploration," which are overwhelmingly advantageous in the low-light, no-light environments of the deep sea.