The sea is simply not flat nor featureless. It is CURVED. Because the Earth is round, don't you get it? You cannot see what would be under the ground because of it.
The sea reflects clutter to the radar. Sea waves reflect radar, and back to the receiver. This creates noise and interference. Its like the sea is an ECM blanket of its own. Naval radars have to be extensively modified from their land based counterparts because of this. Even then, on certain areas, like littoral areas things underneath the water can reflect radar back and create readings.
Why do you think stealth was developed for ships, or at least partial stealth? If you are in AEW aircraft, from a far distance and with the Earth curvature considered, the ship is actually tilted back, and isn't standing straight up. If you increase the sloped angles of the ship on the side, that further steepens the angle of deflection, given you account for the earth curvature geometry. At the same time, the radar waves of the aircraft cannot reach the hull of the ship as it is too low and obscured by the curvature. That's why you see ships like the Shandong and the Type 075 have radar deflection features on their island but didn't have to bother on the hull.
Why do you think many ships have all these long flat sides that has a creased line and angled from above the crease line? Do you think that's for aesthetics? This design feature you can see from ships in France to Russia to China and across the world. Do you think that somehow they are all wrong? Every modern warship nowadays is a practice of partial stealth, with the Type 022, both LCS and the Zumwalt edging towards a more extreme practice.
The Type 056 itself is an exercise in partial stealth, something the Chinese Navy started with the 052B, and is refining with every new design. It also has a low silhouette, so its not going to show up so quickly.