What to do with your valuables on a low-cost holiday while swimming/diving in Central America?

My method is to keep valuables in a piece of luggage that locks. It won't stop seriously determined thieves from slashing open your luggage, but if they're that determined, they could probably break open a cheap safe, too. For documents/money and other small items, I try to hide them on a hidden or hard-to-access interior pocket within my locked luggage so it's not visible immediately upon opening.

For things that I must keep on me (keys, mostly), I use Jonik's method of bundling them in my clothes/towel. I also try to set up on the beach at least a few meters away from other people so that I can notice more easily if someone is lingering near my stuff. (Not possible to do on some beaches that are small and crowded, but I do it when I can.)

In general, don't be paranoid as it spoils the fun!
Here are some specific tips I've gathered over the years!

At the Hotel

1. Ask the owner to take valuables such as passports into safe keeping.
2. Lock your stuff inside your baggage and strap your baggage to the bed - It might slow down an opportunist enough to reconsider. If they have to break in to your luggage you have proof for insurance
3. Take the room key off the fob. It makes it easier to hide the key and if it is stolen they will not be able to find your room so easily.

At the Beach

1. Make friends with nearby people. Keep an eye on their stuff while they are swimming and ask them to return the favor.
2. You can buy waterproof neck wallets from supermarkets and airports for a few pounds. They will keep credit cards / keys / cash damp but safe while you swim.
3. Consider a waterproof camera/case so that you can take it with you while you are swimming.

When Diving

1. Get the dive shop to keep your valuables.
2. On the boat, keep your stuff out of sight in a small pack and keep secured so it doesn't go overboard. Divers are a pretty close-knit group I've never heard of anything getting stolen from a boat at sea.

Twisted's suggestion of taking the keyring off any hotel/hostel keys you get is a great one. They're always bulky to carry and they both provide access to and identify your hotel room. Very bad!

Here's a clever idea for hiding your valuables in clear sight while you're taking a dip. I imagine it would do a great job of preventing opportunistic theft which is often the most common.

Also, studies have found people are far more likely to intervene and stop a thief if asked to mind the victim's stuff. We're also more likely to help people we see ourselves as similar too or empathise with. So if you do have to leave valuables unattended, find someone who looks like you and ask them to keep an eye on your stuff!

Pac-Safe makes wire mesh bags to make it harder for someone to steal your stuff. You lock the bag around your stuff and to something substantial. I have never actually tried them to know how they work, though.

For the larger stuff that you don't necessarily need to always have with you, I'd keep them in a safe at the hotel room. Even at cheap hotels most now have a digital safe. When the room doesn't have a safe, I sometimes ask the hotel concierge to look after it.

For room keys, wallets/money and other really small items, I keep them in a waterproof component in my bathing suit. This means I don't have to leave them unattended on a beach.

My wife and I used an Aquavault, secured it right onto the boat rail or rope when we were out diving, found it online on Amazon, cool product, great quality.