I just had a thought after buying a couple of money belts for my girlfriend and I.. Where should we keep them when we are swimming for example?
I know this seems like a simple problem but is there any other solutions other than taking turns swimming? i.e Burying them?
Since your question is tagged with backpacking and wild-camping, I am assuming you are out in the wilderness.
In that case, just hide your valuables prior to arrival at the beach. Just head 15+ feet off of the trail, and you should be trivially able to find a spot to hide a handful of stuff.
From geocaching, even if someone was to know the general vicinity of where something was, it can be quite a challenge to locate a fist-sized cache of stuff amongst everything else around.
When I've traveled, I've never had a problem finding a random bunch of rocks that I can move to expose a little hole in which to tuck my stuff, then cover them back up, and I've even done that in busy, popular places without any problems.
To ensure your valuables stay dry and clean (wouldn't want to short out any electronics like your car remote lock or phone if an unexpected shower comes along) you can use a waterproof bag (relatively inexpensive) or an Otterbox (or similar) - both found in kayaking section. To ensure that you are able to find your items that you have so cleverly hidden (especially if it is a busy trailhead or high traffic area) especially if you are in the back country for 10 days or more. (I've come back curious to where I'd even parked;-) and you are one that uses a Hiking & Trekking GPS Unit use this to mark the secret stash. This way you will not outsmart yourself!
This isn't a direct answer, but a relevant anecdote.
Many years ago in college I went with some friends to a nearby state park with a lake. After a while, we decided to walk around the lake a bit away from the crowds at the official lifeguard-patrolled beach. We got to a nice spot to get into the water at the side of the lake and went swimming. We just put our stuff down at the side of the trail and went into the water. There were occasionally people going by on the trail, but we didn't think anything of it.
When we got out of the water, two of my friends found their stuff was gone. They both had bright-colored backpacks. At first I thought my stuff was gone too, but then realized my pack was right where I left it only about 1 m from where theirs was. The big difference was that may pack was a dull gray-green color and blended in with the surroundings. The theives must have been right on top of it at one time, but apparently didn't see it.
So the lesson is camouflage works.
