Why I Started Building Tethys app

I never planned to build an Android app for a dive computer.

Tethys app started for a much simpler reason: I wanted to keep using the dive computer I already owned.

Why I Chose COSMIQ+

When I bought my COSMIQ+ Gen 4, one of the things that attracted me to it was its simplicity.

I didn't want a dive computer that required cables, special adapters, or a PC every time I wanted to manage my dives.

COSMIQ+ offered something that felt modern and convenient.

The dive computer connected directly to a smartphone over Bluetooth. The Deepblu app could be used to configure the device, synchronize dives, and keep a digital dive log.

Dive. Get back to the surface. Connect your phone. Sync your dive.

Simple.

I am not a particularly frequent diver, so this workflow suited me perfectly. I didn't need complicated desktop software or an elaborate setup. My phone and my dive computer were enough.

And that convenience was part of what I believed I was paying for when I bought the COSMIQ+.

Then the Software Disappeared

Years later, the situation changed.

In December 2023, Deepblu announced that it was taking its servers offline. The company explained that the number of paying subscribers was no longer sufficient to cover the increasing cost of maintaining its server infrastructure.

Eventually, logging into the Deepblu platform stopped working. The mobile application also became unavailable through the normal app stores.

The COSMIQ+ itself didn't suddenly stop working.

The hardware was still there.

The display worked. The sensors worked. It could still be used underwater.

But an important part of the experience it had been designed around was gone.

And that distinction matters.

Deepblu's own documentation described the mobile application as necessary for adjusting the majority of COSMIQ's settings. It was also the convenient way to synchronize dives between the computer and a phone.

Without that software ecosystem, a perfectly functional piece of diving hardware had lost a significant part of its functionality.

That Didn't Feel Right

I know that online services don't live forever.

Companies disappear. Products are discontinued. Servers cost money to operate. Software eventually becomes obsolete.

I understand all of that.

But as an owner, it was still frustrating.

I had paid a considerable amount of money for a physical device that continued to work perfectly well, yet I could no longer use it in the way it was originally designed to be used.

For me, that felt unfair.

Not because I expected Deepblu to maintain its servers forever.

But because the useful life of hardware shouldn't necessarily end when the service behind it disappears.

I didn't want to throw away a working dive computer.

I didn't want to buy another one simply because its companion software was gone.

And I didn't want to depend on an old APK indefinitely just to keep using functionality that the hardware itself was still capable of providing.

So I started asking a different question:

Can I communicate with the COSMIQ+ myself?

The First Connection

At first, this was mostly curiosity.

COSMIQ+ communicates with a phone over Bluetooth Low Energy. If the original Deepblu application could talk to it, there had to be a protocol behind that communication.

So I started investigating.

I connected my COSMIQ+ Gen 4 to my own Android application.

Then I started experimenting with the communication between them.

One step at a time, the computer began responding.

Eventually, I managed to retrieve dive logs from it.

That was the moment the idea changed.

This was no longer just an experiment.

The COSMIQ+ wasn't dead.

It could still communicate with a modern Android phone. Its data could still be retrieved. The missing piece was software.

And software can be rebuilt.

Tethys app

That experiment became Tethys app.

My goal is to build an Android companion application for COSMIQ+ that restores the experience that made me choose this dive computer in the first place.

I want Tethys app to eventually provide three main things:

COSMIQ+ management — connect to the dive computer and manage its settings directly from an Android phone.

Dive synchronization — download dives from COSMIQ+ over Bluetooth without needing a PC.

A dive logbook — keep and manage dive history directly on the phone.

Some of this is already working.

Tethys app can connect to my COSMIQ+ Gen 4 and retrieve dive logs. Dive data can be stored locally and exported.

Reverse engineering and testing are still ongoing, particularly around device settings and compatibility.

There is a lot left to do.

No Cloud Required

At least for now, I don't plan to build Tethys app around a mandatory cloud service.

Dive data is stored locally.

There is already an option to export it.

That decision is partly a consequence of what happened to Deepblu.

An online service can be useful, and perhaps Tethys app will offer optional online functionality someday. But I don't want the basic ability to communicate with your own dive computer to depend on whether my server still exists years from now.

The device is yours.

Your dive history is yours.

I think the software should respect that.

This Is Just the Beginning

Tethys app is still under development.

My only physical test device at the moment is a COSMIQ+ Gen 4, so I am not going to claim compatibility with every COSMIQ generation until they have actually been tested.

There will be bugs.

There will probably be unexpected differences between generations.

There are still parts of the COSMIQ+ communication protocol that I need to understand.

This blog will document that process.

Not as a highly technical reverse-engineering journal, but as the story of trying to give a useful piece of diving hardware a second life.

I'll share what works, what doesn't, what I discover, and how Tethys app develops along the way.

And eventually, I hope to make Tethys app available through Google Play.

Do You Still Own a COSMIQ+?

This is also one of the reasons I'm starting this blog.

I don't know how many COSMIQ+ computers are still being used.

Maybe there are only a handful of us.

Maybe there are thousands sitting in drawers because their owners assumed there was no longer a practical way to use them with a phone.

I'd like to find out.

If you still own a COSMIQ+, I'd love to know:

• Which COSMIQ generation do you have?
• Are you still diving with it?
• How are you currently downloading your dives?
• Do you still have the original Deepblu app installed?
• What would you want from a new COSMIQ+ Android app?
• Would you be interested in testing Tethys app in the future?

For now, I'm building and testing Tethys app with my own COSMIQ+ Gen 4.

But I hope it won't remain an app for just one dive computer and one diver.

The hardware still works. The software ecosystem it was designed around does not.

That's why I'm building Tethys app.