The dangerous undertow means that swimming is not allowed.
危险的逆流意味着不允许游泳。
2a tendency or feeling that seems weak, but in fact has a strong effect 潜在的倾向[特质]
undertow of
The humour of the novel cannot hide an undertow of sadness.
小说的幽默掩盖不住暗藏的悲伤。
Examples from the Corpus
undertow• The wave had a great deal of undertow.• Sylvie had raved, raved with that undertow of intensity which always made her seem to be speaking the truth.• Regardless of the undertow of danger, Johnnie would not give back the diaphragm case.• But it's the undertow of precocioussexuality, the child-woman come-on, that's more worrying.