If you drink from the same sea each time, you should be fine.

But if you were to go to other seas, with different biological profiles, your immune system might not be able to handle it as well.

It's the same concept as vaccination. If you survive it once, you'll survive it the second time much easier, because you already know how to fight it.

But a person new to the area might get sick, although only sick the first or second time, before they get used to it too.