if something is removed from something else then that something else contained that something & water has salt removed ->  water that has salt removed contained salt  salt water contains salt & salt water is a kind of water ->  salt water is a kind of water that contains salt  water that has salt removed contained salt & salt water is a kind of water that contains salt ->  water that has salt removed was salt water  water that has salt removed was salt water & seawater is a kind of salt water ->  water that has salt removed may have been seawater  a sea is a source of seawater & if something comes from something else then that something else is a source of that something ->  seawater comes from a sea  water that has salt removed may have been seawater & seawater comes from a sea -> water that has salt removed may have come from a sea