Li Du understood Hans's personality: he was the type of person who wouldn't be convinced until he'd seen the grim reality for himself, so he allowed him to dive into the sea to have a look at the situation.
For safety purposes, Li Du and Brother Wolf accompanied Hans. Li Du roughly knew the position of the undercurrent while Brother Wolf was a good swimmer and had undergone formal snorkeling training. Furthermore, as Brother Wolf had carried out diving missions before, normal undercurrents wouldn't be able to trap him.
With Li Du and Brother Wolf accompanying Hans, the three of them jumped into the sea and propelled themselves into deeper waters by kicking their flippers.
After Li Du gradually reached the vicinity of the undercurrent, he stopped and motioned to Hans to continue diving deeper.