This one is a bit tough because the Greek is a little idiomatic:
- And the sea wind great blowing was awakened. (interlinear)
- And the sea arose by reason of a great wind that blew. (KJV)
- The sea became rough because a strong wind was blowing.(ESV)
- Soon a gale swept down upon them as they rowed, and the sea grew very rough. (NLT)
- The sea began to be stirred up because a strong wind was blowing. (NAS)
- Then the sea became choppy because a strong wind was blowing. (my translation)
The Greek in this one was tough for me to understand because the order was so wacky. Put into a decent order, the transliteration reads: "And the sea was awakened [by] the blowing of a great wind." The easy change is to convert "the blowing of a great wind" into "a strong wind was blowing".
The harder bit is figuring out what to do with "the sea was awakened". While this is understandable if you put a bit of work into it, it isn't very readable. The sea being awakened is nice poetic imagery in English, but it is hardly common and will puzzle most readers for at least a few seconds. However, this was probably a very common phrase in Greek, possibly an idiom.
So the challenge was how to translate this somewhat idiomatic phrase into readable English. In the Greek, the sea is both passive and personified. Though I racked my brains, I couldn't think of an English phrase that captured both of those. Abandoning that line of thought, I went to English idioms. I steered away from ones that would not be understandable in other languages and stuck with ones that would merely take some work to understand in other languages. I came up with "the sea became rough" and "the sea became choppy". (Note that the ESV and the NLT apparently followed the same reasoning and went with "rough".) I felt that both were pretty accurate to the meaning of the original. However, they have slightly different nuances. "Rough" waters are more dangerous than "choppy" waters. And in comparing the John account with the parallel accounts in Matthew and Mark, John seems to go out of his way to omit any sense that the disciples were in danger. In John's account, the "awakened sea" is merely a hindrance which explains why the disciples were at sea so long, but not a threat to their wellbeing. In this context, "choppy" seemed a better choice than "rough". Had this phrase occurred in Matthew's or Mark's account, I would choose "rough". (Note: if I had chosen "rough", my translation would have been virtually identical to the ESV for this verse.)