For me, water wicking through the fabric and the holes stitching makes has always been a concern for me. If you are quilting through all the layers, I don't think that you can make it fully block any water leaking through. You might experiement with gluing on a thin layer of cork that you get at craft stores on the back after it is done, but that would ruin the look of your quilting on the back. You might also want to try using two layers of bat with a layer of thin vynal that you get at Joanns in the middle. That would let the top bat absorb water and the second bat absorb any leaks. You may want to try different ways and pour some water on them and see if any or how much water leaks through. I know that people use quilted coasters all the time, but I only use coasters that I know will block 100% of the water on top of my good wood furniture.