Throughout your search, you may run into the term of welded seams. As it sounds, it is a sufficient and superior way to ensure that the fabric is safe from penetrating water. How it works is it uses radio frequencies to merge two separate pieces of fabric together, bonding them to be one whole piece of fabric. As compared to sewn-and-taped seams, this is a far more effective method.
With the way that sewn-and-taped seams work, two pieces of fabric are held together with thread and protective tape. While this is not necessarily an incompetent way of constructing a dry bag, it certainly does not provide the protection that welded seams do. This can withstand two times more stress than that of sewn-and-taped seams. In other words, when you see “welded-seams”, you know that it has superior craftsmanship.