What stings in the water at Nha Trang beaches and how to protect yourself?

Sunscreen is useless once you're in the water — it washes off almost immediately, so only protective swim clothing (rash guard, leggings) actually works, even if it's a bit of a hassle.

At Vinpearl beach, something stings the legs while swimming — you feel a burning sensation that fades after getting out of the water; wearing a rash guard/swim top reduces stings on the arms and body.

Long hours in protective water shoes can cause painful blisters — bring blister patches with you, otherwise walking becomes very painful.