How to Mark a Dive Site's GPS Coordinates and Log It Right
To mark a dive site by GPS, save the coordinates topside, on the surface, while the boat is sitting directly over the spot. GPS signals do not reach underwater, so the mark has to happen before you drop or right after you surface, never on the bottom. Idle the boat until your sounder shows the reef or wreck under the hull, drop the pin, then log the depth, the entry, and the visibility. That gives you a spot you can run back to next season with zero bars.
The coordinates are the easy part. The work is capturing them fast on a moving boat, then saving the conditions that tell you whether the site is worth diving on a given day. Here is how to do both.
Why dive sites are hard to relocate
A dive site sits under 40, 60, 90 feet of water with nothing on the surface to mark it. From the boat it looks like every other patch of blue, and even a wreck the size of a freighter leaves no wake up top for you to line up on.
The boat does not stay put either. You cut the engine over the reef, then spend ten minutes rigging tanks while the wind or current walks the hull a few hundred feet off. The coordinate you eyeballed on the way in is now nowhere near where you actually are.
Current pushes you off underwater too. You giant-stride over the site, drop down, and by the time you hit the bottom the flow has carried you off the structure. Divers surface a hundred yards downcurrent all the time with no idea which way the boat is. All of this is why a saved coordinate beats a memory of "it was off that point somewhere."
Mark the dive site's GPS coordinates topside, before you drop
GPS is radio, and radio does not travel through saltwater. The second your phone or handheld goes under, it stops fixing. So the mark has to be made on the surface. There are two clean ways to do it.
- Mark from directly over the site. Idle the boat until the sounder shows the reef or wreck under the hull, then drop the pin. That lands the coordinate on the structure itself, right where you want it next time.
- Mark your entry, then send a down line. On a live-boat or drift setup, pin the spot as you splash and drop a weighted line so the group has something to descend on and follow back up.
A phone fixes your position to within about 10 to 16 feet under open sky, which on the water puts you close enough to find the structure on the sounder and get a line on it. A dedicated handheld does about the same. Whichever you use, save the pin before the dive, while the boat is settled. Trying to recall the numbers after an hour on the bottom and a surface interval is how good sites get lost.
What to log for each site: depth, entry, viz
The coordinate gets the boat over the site. The notes tell you whether to dive it. Three things earn their place in every entry.
- Depth. Note the max depth and the depth of the top of the structure. A wreck whose deck sits at 60 feet and whose sand is at 95 changes your gas plan and your no-deco time. Log the thermocline if you hit one, because that cold layer at, say, 45 feet is often where the viz drops out.
- Entry. Write how you got in and where. Giant stride off the port side, negative entry straight to the line, back roll from the tender. Note whether you dropped on the structure or had to swim to it, and which way the current ran, so the next crew splashes in the right place.
- Visibility. Green 15-foot murk and 80-foot blue water are two different dives. Log the viz you got and, over a few trips, the pattern: clear on the incoming tide, blown out for two days after a hard blow.
Water temp and whether there was any current at depth are worth a line too. Ten seconds of notes on the boat saves you a wasted run six months later.
Marking wrecks and structure you can't see
A wreck or a patch of reef is a small target you cannot see from the surface, so you find it with the sounder and mark it the instant it shows under the hull. Run a slow search pattern over the published area, watch the bottom machine, and when the profile jumps up off the flat, that is your structure. Pin it right then, before the boat slides past.
If you are working from numbers someone handed you, or from a published wreck list, treat them as a rough box to search, then confirm the mark yourself. Coordinates get copied wrong, chart datums differ, and wrecks get dragged by storms and trawls. Run to the area, find the mark on the sounder yourself, then save your own pin on the real return. Watch the coordinate format while you are at it: the same spot written in degrees-minutes-seconds versus decimal degrees can land you a mile off if you mix them up. Your own fix is the one you trust next time.
Getting back to the same site next season
A saved coordinate is good for years, but the diving around it changes with the calendar. When you save the pin, save the context: the month, the tide stage, the water temp, the viz you got. Next spring the numbers still run you to the same reef, and your own notes tell you whether early June is too soon for the clear water or right on time.
For anything that depends on flow, log the tide. A lot of wrecks and reefs only dive well at slack, the short window between tides when the current stops. Note the time you splashed relative to high or low, and you can plan the next trip around the same slack instead of fighting a three-knot rip. Do the same for the good stuff you found down there: the swim-through, the ledge the big grouper holds on, the anchor chain that leads to the boiler. Put those in the log while they are fresh, and the site stays a known quantity.
Keep your dive log on your phone
A dive site you worked to find is worth keeping to yourself. Post a wreck's coordinates and by next season it is a cattle boat over the same deck. Keep your marks somewhere that stays on your phone instead of on a company's server, leave the location tags off your photos, and when someone asks for the numbers, keep it general. There is no feed to leak them to and no account with your name attached. A spot only you can run to is a spot that stays good.
Common questions
Does GPS work underwater?
No. GPS uses radio signals that do not pass through water, so your phone or handheld stops fixing the moment it goes under. Mark the site on the surface, either before you drop or after you surface.
How accurate is a phone GPS for marking a dive site?
About 10 to 16 feet under open sky. That is close enough to run back to the spot and pick the reef or wreck up on your sounder, then drop a down line on it.
How do I find a wreck from GPS coordinates?
Run to the area, then search with your depth sounder until the structure shows under the hull. Save your own pin on the real return, since published numbers are often off by a datum difference or a storm's worth of drift.
Can I save dive sites offline with no signal?
Yes, if your app stores the map and pins on the phone. Save the area once at the dock and you can drop and open pins with zero bars out over the site.
One tool that does this
Wakelog is a private logbook built for this. You drop a pin in two taps and tag what it is (Dive, Anchor, Snorkel, Fish, Sandbar), then add the depth, entry, and viz in the notes. When you want to go back, its Take me back feature points a live bearing and distance to any saved spot. It points; you drive, and you still navigate with proper charts. It works offline with zero bars, everything stays on your phone with no account and no feed, and you can export every spot to a GPX or JSON file, so nothing is locked in.
It's on the App Store for iPhone now (14 days free, then $12.99 a year or $49.99 once for life). If you want a private record of every reef and wreck you dive, download it at getwakelog.com.