As others have said, the biggest culprit is probably not basting the sides. If you float the top you should still baste the sides as you go - just run a quick line up each side every time you advance the quilt. This also helps keep it square.
From the amount of waving, I think you might have also had it too tight and/or had the foot too close to the quilt top. Is your batting quite thick/puffy? If it is, you might want to have things a bit looser than usual, so that the top isn't too close to the foot. A millimetre or two can make the difference between the foot gliding nicely over the top versus pushing the top out of place as you stitch.
That's a super cute quilt! I would definitely keep the waves - I think they look cool.