"Water certification" refers to a process where someone or something is recognized as meeting certain standards or requirements regarding water-related knowledge, skills, or quality. Full definition
To earn your PADI open water certification, you'll need to perform a number of confined dives and then four open water dives. (uponarriving.com)
If diving, you will have to provide proof of open water certification from a nationally or internationally recognized organization, along with photo identification. (georgiaaquarium.org)
Only open water certification required for this tour, not cave certification. (cancuntours.net)