Do you want to hunt the hippo out of the water or not? If you would like to try and take one on dry land, I would consider a .416 something as a minimum. A .416 is fine for Buff as well. It is probably less than ideal for croc, given the typical distances involved. A lighter weight solid may do the trick for croc, if you can develop a load that shoots close to MOA. (Your "rapier" analogy is spot-on as it relates to croc.)