30m is really just for rain, humidity, and light force accidental splashes. Probably not ideal for swimming. If you want to swim, I'd probably recommend that you use the versions with 100m water resistance. Which would be good with a freestyle stroke or butterfly stroke surface swimming and some snorkeling, but not any major diving, nor jet ski activities. Here's a suggested table from Casio that's consumer friendly. www.casio-intl.com