I never hunted hippo, nor elephant.
But this discussion is actually the same as many other on the forum, how ethical is to hunt large game, with smallest possible caliber, and accurate shot placement.
This debate is ongoing since invention of smokeless powder and small bore rifles in the range 6.0 -6.5 to 8 mm, the debate fueled by 100 year old writings of Bell.
There is also available on this forum, few pages from an article in man magnum magazine, where a research was conducted, based on courts ruling and court evidence records where a poacher was sentenced for poaching elephants with 22 rifle. So, this all suggests anything is possible with any caliber.
Possible, maybe yes, but not recommended.
My personal view: for all dangerous game, to use caliber as large as possible, and personally, I would not shoot hippo with anything else less then 375 H&H, in water or out of water.