Take your favorite picture of a beach scene (perhaps one with you or the person you're making this for in it), print it off on Printed Treasures or some equivalent and use that for your last block.
This happened once to a lady on retreat that was making a baby blanket. The blocks didn't come out right to make it the size she wanted, she needed two more blocks. So she did one with the baby's name, dob, wieght, length, etc. and another with the parents/siblings' picture. It turned out beautifully.