John Gill Commentary John 6:18

"And the sea was rising by reason of a great wind that blew." — John 6:18 (ASV)

And the sea arose
Swelled, and was tumultuous and raging; the waves mounted up, and tossed the ship to and fro:

by reason of a great wind that blew ;
which agitated the waters of the sea, and lifted up the waves; which storm seems to have arose after they had set sail, and were got into the midst of the sea.