Probably a question that has been answered countless times already but do manufacturers have to verify water resistance claims somehow?
Very informative. Thanks.Yes and no.
They don't "HAVE" to, and many don't. Some estimate WR based on engineering specifications and computer simulations. Others do dry pressure tests on a few cases as they come off the line. Others adhere to higher standards like ISO 6425 (see here for the wikipedia entry on ISO6425). Most of your big names do adhere to ISO standards or better, but some companies make outrageous claims to water resistance that I doubt could be backed up.