Tell us about yourself!
Mommajozy is right use what you want.... you could have cut and sew a number of towels together to make the back.... For the use of your quilt on the beach you do not need batting. You could put the top and back together and quilt them as the tshirts and towels can be heavy.
Hope you don't mind me helping.You can use any type of blanket for batting when making a quilt for your own use. I would caution however that you do not use one if it has thin sections or the inside batting is shifting. Shifting batting will give you thick and thin sections all over your quilt.
Good Luck hope your quilt turned out nice.