If you don't drink natural water from a single source often, I recommend boiling it, or subjecting it to UV radiation, or even using it in an alcoholic drink, rather than just drinking it.
There could be many pathogens within the water which could cause harm if they're not dealt with.
My stomach seems to deal with them just fine, but thank you very much for your concern. When I drink sea water, I take it from far away from the next city, that is my only concern.
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.
Good point. Never thought about it. This is the first ocean I drink from. :)