Probably nothing that's featured on the lockpicking lawyer's channel

It's an entertaining channel about locks and lockpicking, but many of the most popular videos are about highly insecure locks that he defeats in seconds, especially if he can do it without specialised tools. He opened a "safe" with a fork on one video and people jokingly challenged him to open a "safe" with a spoon. He succeeded in doing that. Some "safes" don't deserve the name as the contents are very far from safe.
More seriously, my quick looking around indicates that BS/EN 1047 is the standard in the UK and in the EU for safes. Mainly in terms of heat resistance, though I think there's some assessment of security from unauthorised access too. Water resistance is a different thing. Maybe you'd be better off sealing the contents from water before putting them in the safe, since they'd be very vulnerable to water damage.
Also, as BigT mentioned above, for security from theft you also need to ensure that it's impractical for a thief to take the safe away. If they can remove the safe and take it to somewhere they can spend some time to try opening it, they might well do so.