When I want to come back to a spot where I saw something remarkable like a seahorse, I really wish I had GPS. But the problem with GPS is that you can’t get the signal underwater. At least not without having to spend USD$3,000 on a cumbersome GPS device with an umbilical cord to the surface.

But Mark Thome has figured out a cheap and simple way to mark the spot you want with just the simplest GPS handheld. Here’s how:

1. Make sure your GPS handheld is able to retain your last location on screen before it loses signal. Most GPS devices already do this.
2. Put the GPS device in an air-filled, air-tight and transparent container. Pad the container so that the device doesn’t move around and accidentally turn itself off.
3. Securely tie a line with a reel to the container.
4. Take the whole set-up with you underwater, but leave the device turned on. Make sure it has enough battery to last the entire dive.
5. When you want to know the location of a spot you are at, let go of the container with the device, reel it slowly upwards and make sure it hits the surface.
6. Wait a few minutes while the device obtains a signal and registers your location.
7. Reel the device back down. Record your location on a slate.
8. Voila! Now you know where to find your secret dive spot.

There are some obvious drawbacks to this method, depending on how deep you are and the strength of the current above you.

The stronger the current, the less accurate the reading, since your GPS buoy will act like a sail on the surface and drag itself off in one direction. And if the current is very strong, you run the risk of being dragged along the bottom or your buoy breaking off or snatched from your hands by the current.