We want to be able to get out of the sub in high depth to photograph and measure wrecks. With usual diving equipment you are very limited down there.
From a so called "open scuba-gear" you are getting your breathing gas at the same pressure as your environment, to keep your lungs "ambient", that means equalling the pressure with the environment. Due to this fact the gas consumption per breath increases significantly in higher depth.

Is a lung in 1 bar environmental pressure being filled with five liters of air, this amount increases in 100 metres depth and 11 bar to: 11 x 5 liters = 55 liters of gas volume. In 250 m then 130 liter per breath. A 10 liter bottle contains 10 liter x 22 bar = 2200 liter, useable in depth are about 2000 liter. That divided by the 130 liter per breath results in just 15 breaths. Converted to a portable 2x15 liter bottle combination the result is 45 breaths. This calculation is just an example and very simplified, but it shows, that you won't get along well in high depth with usual diving equipment.

The solution are closed rebreathers. Here the gas is being breathed in a circle and only the used oxygen has to be replaced. The operating time for those devices is several hours, and that mostly depth independent. Furthermore you are being kept warm, as you are breathing your own, warm exhalation gas.
The disadvantage of these complex devices is, that all devices on the market do not have a "backup", a compensation system, in case the primary device is failing. Most of the devices switch to the open circle, that means, you breathe directly from the bottles - but as you saw, they are (as the above calculation shows) immediately empty. So no real solution for safe diving in high depth - and consequently not for our project!

So we began to deal with "rebreathers", old military equipment but then we looked for an own solution. This solution is named EXIVE in the meantime and is a rebreather with fully integrated backup cycle. 6-8 hours diving in nearly any depth and a fully qualified backup cycle ensure safe diving.
This novel unit together with the decompression chamber aboard the sub allows us, even in high depth, to dive safely and autonomous - and then desaturate by hours or days.

The scooters on the page are only an "additional" hobby.