If the anchor was lying on the bottom, gravity would no longer be playing a part in the equation...not for that component anyway. The anchor would however displace a given volume of water, causing the level to rise.
Now, due to the weight of the anchor having been deducted from the boat, the displacement will be less, causing the water level to decrease.
Without knowing the ratio of weight to volume, we can't even start to calculate the net effect.
