We do not know how salty the sea was before the Flood. It may have been less salty than it is now. It may have been more salty. Volcanoes emit huge amounts of steam and hot water. Many estuarine and tidepool species are able to tolerate wide changes in salinity.
Barnacles can withstand exposure to less than one-tenth the usual salt concentration of seawater.
There are migratory species of fish which travel between salt and fresh water. For example, salmon, striped bass and Atlantic sturgeon spawn in freshwater and mature in saltwater. Eels reproduce in saltwater and grow to maturity in freshwater streams and lakes. Indeed, most of the extant orders have both fresh and saltwater representatives. This suggests that the ability to tolerate wide changes in salinity could have been present in most fish at the time of the Flood.