There are two effects of throwing the anchor into the pool.

• the anchor will raise the water level, since it will displace water equal to its volume
• removing the anchor from the boat will lower the weight of the boat, reducing the amount of water displaced by the buoyancy of the boat

Recall Archimedes' principle, which states that the weight of the water displaced is equivalent to the weight of the boat. Since the anchor is more dense than water, the amount of water the anchor displaces at the bottom of the pool is less than the amount of water displaced by the anchor being in the boat. As a result, the level of the water goes down.