Salt water is highly conductive, and also corrosive. Electricity speeds the corrosive effects of salt. Electrically accelerated corrosion could burn through the copper traces on the board in rather short periods of time. The Lithium Ion battery can dump large amounts of power very quickly. With salt water to conduct the electricity damage can happen very quickly.
If it were my phone, I would disconnect the battery, remove it, then bathe the phone and motherboard in a solvent that can remove the salt without further damage to the board or case. Frankly water may be the best solution since salt dissolves in water.
Once the salt is completely washed away, then it's just a matter of thoroughly drying it, replacing the battery and attempting to boot. If you don't get all salt removed, it may work for a while but could eventually fail. Of course, it may never work again anyway.
If you have insurance on it that covers water damage, place a claim and be done with it.
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