I went to the seaside but I was not allowed to bathe for they said that the sea was too rough.

rough 是形容词，本义是"粗糙的、不平整的"，在这里引申为形容海浪"汹涌的、狂暴的"。用来描述海面状况时，rough 特指风浪大、水面不平静的状态。