Some of the terms you are looking for are density and buoyancy.
The quick and dirty explanation is that the more dense something is, it will naturally sink below less dense material and float on top of more dense material.
Buoyancy, in layman's terms, is related. This is what allows a ship that is made of steel, which is much more dense than water, to float. Buoyancy is kind of like relative density. A ship floats because all of the mass is spread out over enough surface area to make the ship, as a whole unit, less dense than the water.
So now for the animals. I am a 117 kg Animal and I float fairly well, that's because I am fat and therefore, as a whole, less dense than the water. My son is a very muscular 90 kg Animal with almost no body fat. He is, as a whole, more dense than the water and is not buoyant. Hippos are so very mobile in water because they are fairly close to being the same density as water. They are slightly buoyant. Tigers, being heavily muscled, are less buoyant but are very strong, flexible, and agile. Therefore they are very strong swimmers. (incidentally, hippos are pretty fast on land too, and they are NOT friendly. They kill a lot of people.)
They animals you intend to drown aren't likely to make it easy for you unless they have the following characteristics:
Negative Buoyancy. They are more dense than water, and not by a trivial margin. This is a potential problem for large, very strong creatures.
If they are flexible and agile enough they might be able to overcome the negative buoyancy. If they are strong, but with limited mobility, they are going to have a hard time in the water.
They would have evolved in a place that has no major bodies of water, or at least nothing deep enough to be a persistent threat of obstacle.
Apologies to any biologist tearing their hair out over this very simplified version.