Overall composition of a planet is what would matter, not whether there is land. If there is salt on the planet, it would almost assuredly have salty oceans. Salt diffuses in water. If you put salt into a glass of water and leave it sit, eventually the salt would dissolve and mix completely. Salt water has a different density than water. The act of dissolving involves energy changes. These create small eddies and currents that would mix the water until it was in equilibrium. If there is salt in any form on your waterworld, the only way it wouldn’t be salty is if the salt was permanently separated from the water physically.