While option 4, "Emphasizes the wetness of the sea," is a reasonable interpretation of the line, the most direct effect of the personification in "The sea was wet as wet could be" actually suggests that it might be highlighting the sea's inherent quality of being wet. However, the use of this phrase can also evoke a sense of liveliness in the sea, giving it a vivid character.
Overall, the phrase primarily emphasizes the extent of the sea's wetness, making option 4 a suitable choice. However, consider that option 1 ("Makes the sea seem alive") could also be a relevant interpretation as the phrase gives the sea a dynamic quality. Thus, both interpretations have merit depending on the context in which they are analyzed.