This question assumes the oceans were salt water during the Flood like they are today. Many creationists believe the entire world was largely fresh water. Today about 30% of the rain water washes into the oceans, bringing mineral salts with it. The oceans are getting saltier every day. Today’s oceans are about 3.6% salt. Between the salts washing in from ground water and the salts leaching in from subterranean salt domes, the oceans could have gone from fresh water to 3.6% in the 4,400 years since the Flood. If the earth were billions of years old, the oceans would be much saltier—like the Dead Sea or Great Salt Lake.
Animals Adapt to Salt Water
Many animals have adapted to the slow increase in salinity over the last 4,400 years. There are now fresh-water crocodiles and salt-water crocodiles that are different species but probably had a common ancestor—a crocodile! This is not evolution. It is only variation. Changing from a fresh-water croc to a saltwater croc is not a major change compared to what the evolutionists believe. They think it changed from a rock to a croc! That would be a major change!
Several years ago, a man in Minnesota told a true story of two large aquariums in his house, one fresh water and the other salt water. He wondered if he could mix the fish together so he figured out how to slowly raise the salt content in the fresh- water aquarium a little each week for ten years until it was 1.8% salt. At the same time, he was lowering the salt content in the salt-water aquarium to 1.8% salt. After ten years he mixed all the fish together. He said they adapted fine.
Noah had no problem with drinking water during the Flood because the fresh-water/salt-water problem did not exist. Attempting to force the way the world is today onto the questions involving the pre-Flood world is a common problem. Second Peter 3 says the scoffers of the last days will be willingly ignorant of how God made the heavens and the earth (the original creation) and the Flood.