You need to use 100% polyester thread and apply a seam sealer over the seams. Seam sealer comes either as a tape that you seal on with an iron, or as a glue-like substance that you apply with a brush. Either option is effective, but the liquid is a better choice if your fabric is heat sensitive. Another option would be seam welding, where you heat the layers at the seam so that the fibers melt together at that area, but it takes just the right kind of fabric, with just the right equipment and a lot of practice to get the technique down. I would recomend, since you are putting this object in water, that you use a flat-felled seam reinforced with a zig-zag stitch down the middle. check out the link for details on a flat-felled seam http://www.geocities.com/monstonitrus/a_and_s/seams/seams.html