After drying out an Ocean Monument & doing a thorough search of all rooms, it was apparent it spawned without a treasure room. There were three rooms with sponges (about 95 total), a separate room with a prismarine column in one of the wings, an arrangement of Sea Lanterns and columns plus an arch in the other wing, but no treasure room, no gold blocks. The rear of the central area where the Treasure room normally is found was completely empty floor to ceiling.

Screenshot shows "empty" rear of monument, taken from the farthest rear wall, looking to the front, torches mark structure centerline.