Not quite. The volume of the water is not the same as the weight of the elephant. You'd have to estimate the density of an elephant and multiply that by the volume of the water to get the mass, then multiply that by the acceleration due to gravity in water system (SI, English Customary, etc.) you're using. Luckily, mammals are mostly water (humans are around 70% water on average), so about 2/3 of the weight of the elephant would be equivalent to the weight of the water displaced. So you would have to estimate how dense the rest of the elephant is (since it'd be minerals and such, I'd say it's more dense than water) and follow the steps described above.

Use a pool -> calculate the volume of water of the pool and the water level, put the elephant in the pool, take note of the level after the elephant after it is in the pool. By Archimedes principle, the volume of water dislocated is the same as the weight. So the weight of the elephant is the same as the difference in water volume.