In general eating lychee can cause diarrhoea, but it does not necessarily lead to watery stools.
If you normally have gastrointestinal disorders or suffer from digestive disorders, eating or over-eating lychee may lead to diarrhoea. Because of the high sugar content of lychee, the presence of diarrhoea on an empty stomach or after accompanying hypertonic food may lead to diarrhoea or as a result of eating unhygienic lychee, which usually resolves on its own.