Use electrolysis to separate oceanwater into hydrogen and oxygen. In a separate tank, burn the hydrogen and oxygen to produce water again.
This would leave salt and other residue in the tank where the electrolysis is executed, but not if you keep oceanwater constantly running through it at a slow speed.
This may sound like it would take a heck of a lot of energy, just like conventional desalination, but maybe it won’t if you feed the energy released by the system back into the system in two ways.
1. Use the heat generated by burning the hydrogen, by converting it to electricity, to power the electrolysis happening simultaneously in a different area.
2. Use the collapsing of gas pressure caused by burning the hydrogen and oxygen to lower the pressure where you’re doing electrolysis, to help it along. Since electrolysis has to fight surrounding air pressure to convert the water into gas, and all energy is conserved, it stands to reason that, to some degree, the surrounding air pressure translates to more electricity being used by the electrolysis.
This may require a complex and expensive setup, but the energy saved may just make it worth it in the long run.