Does it matter if you’re baptized in salt or fresh water?
It probably doesn't matter. Acts 8:36-38 describes how Phillip the Apostle baptized an Ethiopian eunuch on the road to Gaza. When they found water, probably at Ein El-Hanniyeh, a natural spring still used today for baptisms, Phillip told the eunuch that all he needed to do to be baptized was to affirm that he believed with all his heart that Jesus Christ was the son of God. So what was important was not the water but what was in the man's heart.
Does it matter if you’re baptized in salt or fresh water?
I can't imagine that it would make any difference. When Christ walked the earth many seaside communities would likely have rationed the fresh water for drinking and other purposes and used salt water for baptisms.