One of the things I always wanted to give a try is a semi closed rebreather system.

Compared to a normal open system used in diving where all your air is blown out directly in the water. The rebreather has an exhale and inhalation pack, comparable to an external lung to your body. So the air you just breathed out is contained and the CO2 is removed by a lime cartridge. A small tank with, in my case 40% oxygen is used to replenish your breathing gas. Resulting in a 35% mix, roughly 40% minus the 4-5% of oxygen the Human body is using. This number could change slightly depending on the activity level. On a small 4 litter tank at 200 bar you can stay over 1h30 underwater.

This gives you a limited operating depth of around 30 meters and very long non decompression limits (NDL).

The instructor I was diving with was using a normal air tank, we spent a long time down a 18 meters and he was hitting his NDL and air limit, I could have stayed another hour
It is also a different diving feeling as when you exhale you don't lose that air, meaning you stay more stable in the water and don't go slightly up and down when inhaling and exhaling strongly.

And the best part, you almost have no bubbles coming out, and if there are they are not in a big burst. This is great when you approaching marine life as bubbles are one of the main things that scare them. It makes noise and air is a normal thing for them to encounter.

Now the question is, would I want to buy one... as they are a bit more bulky and requires usually high level of oxygen it makes it a bit harder to travel with. And they are only fun when your dive buddy also has one to get the maximum out of the dive. Will see : )