Sea water is just water, with a higher mineral concentration (salt). Therefore, it is not healthy because minerals are good for us,but not in such concentrations. So that he can drink the water, he needs to somehow separate the salt from the water.
How?
He could use a process called distillation where he would boil the salt water, collect the vapor that comes out and condense it so that he can drink it.
If he was not in anisland though, I would've suggested other more scientifical but easier methods such as using reverse osmosis, which would only work on a lab or with special equipment.
Hope it helped,
BioTeacher101
