Not needing to urinate or very dark yellow urine is a danger sign. Always carry a water bottle with you on long trips. Excessive sweating can lead to loss of salt and therefore muscle cramping. Many health problems can be avoided by taking care of yourself. Avoid excessive sun and always wear a hat as well as sunscreen.
Sunburn is all too easy to get.
I even got a nasty dose the last time I was in Fiji. Wash your hands frequently, it's quite easy to contaminate your own food. Avoid climatic extremes, keep out of the sun when it's hot, dress warmly if it's cold. Avoid potential diseases by dressing sensibly. You can get worm infections through bare feet or dangerous coral cuts by walking over coral without shoes.