Im sure they would do fine in brackish, as for saltwater it's kind of an unknown. I don't think they would thrive long term but it's anyone's guess honestly.
whipers dont get bigger then 30 lbs at max they like coldish water and do well with flow. they swim a lotttt not like large mouths that just lurk. whipers will school up and swim, a large round tank would be best or something with plenty of flow in one direction