Thanks @blinks for telling us about the Sentinelese tribes. I really never knew that such tribe as primitive as this still exist today.
As for your questions, it's important to note that people of such are masters of their environment- the forest. Don't forget that herbal medication is very potent and most of the leaves and trees found in the forest are very medicinal, hence, they will rely on that and never need any modern form of medicine.
Secondly, bearing in mind that they live in the island, it won't be wrong to assume that they're masters in swimming. People of such calibre have very high rate of survival skills and will utilize those skills when they're in danger.
@largerben you have made some very clear and useful points, but there are also other tribes like them who at one time of the order were nearly wiped out from the surface of the earth because of epidemic out breaks and the civilized world had to wave in to resure them by giving them mordern medicines even when they had herbal medicines at their disposal.
Secondly no matter how good one can swim, I don't think one can survive a tsunami. If you ask me I think the sentinelese tribe are mysterious humans or what do you think?