A cell is surrounded by a partially permeable membrane which allows water to move in and out of the cell. Depending on the relative concentration of the water, it will go out of the cell or into the cell by the process of osmosis. Ocean water is hypertonic to the cell. This means that its concentration I higher than the cell.
This is why water has a tendency to move out of the cell. Once it does so, the cell loses its water and shrinks. The cell body shrivels as the cell decreases to a smaller size. If this is continued, the cell will ultimately die.
If an animal cell were placed in a vial of ocean water, the cell would shrivel. This is because the solution outside the cell - the ocean water - is saltier than the solution inside the cell; in science, it’s called a hypertonic solution. When one side of the cell is saltier than the other, the water moves so that the end result is two isotonic solutions - two solutions of equal saltiness. The water inside the cell will leave the cell, resulting in the cell shriveling up. It will not die unless the ocean water had extra salt stirred into it.
The opposite would happen if you placed the cell in pure, unsalted water. The cell would swell because the solution inside is saltier than the one outside.