In terms of total force, it makes absolutely no difference if a being is standing on land or floating in water. A blue whale bears its weight of 140 000kg regardless is it under water or on ground. The difference is just that under water the weight gets distributed more evenly than on a hard surface.

Gravity will act more strongly on whichever has more mass (thus, more weight) per given volume. So if the thing you immerse is denser than water it will sink, but its apparent weight is reduced by the volume of water that gets displaced upward. If instead the water is denser, the immersed object will float up to the point where the displaced volume of water matches the whole object's mass. Then the net weight is zero.