You have seen them on the dive sites and have no clue what it is or how you can get started?
First of all we need to sort things out. Rebreather diving has been around since 1800. Mostly done by the military divers and with machines that costs fortunes to buy and maintain. The thing with Poseidon Rebreather, is that it was the first aim at the recreational market. It was the big revolution in diving, since the recreational market is 10000 times bigger than the technical side. Together with old school rebreather inventor/diver they came up with the MKVI 2009. It was not a rebreather and will never be one. The difference lays in the monitoring system of the diver. A system for a recreational diver has to be so much more sophisticated than a rebreather monitoring system. A "normal" rebreather can kill you in 2 min if you don't know what your are doing and can respond to different types of errors and changes inside the system. The Poseidon system is monitoring you as a diver and will tap you on the shoulder when something is about to happen. It look 15min ahead in time and tries to predict what will happen. It will then ask you to do something about this. This approach is unique. Other manufactures makes rebreathers. They tell the diver to get training and experience. Even experts do mistakes. On a Poseidon it is more likely to get information about what is going on and the being able to respond correctly. The machine is a little bit more advanced and if you don't look after your machine or have been taught the knowledge about small-little-things-that-can-go-wrong-during-startup then you will hate the machine. A good instructor will teach you all these skills and know hows. A rebreather is a simple construction of lungs, gas, sorb, hoses. This way you can always override thing and get into the water with a non working machine. Poseidon has another approach. If something is wrong on the machine, you wont get it into the water. However, it has a fail safe function. Even if you jump into the water without starting it up, it will start up and keep you alive until you are above water again. A feature no other machine can perform. The Poseidon is the safest machine in the world.
The thing for beginners are to get a SE7EN Sport. A basic startup version of Poseidons life supporting machine. It is priced very aggressive and is also upgradeable to a full Trimix +150m beast. Your choice.
If you are sick n tired of filling your tanks and looking at the manometer all the time, then this is for you. Diving with a Poseidon is a joy. You can stay down 3-4 hours with 3lit oxygen, 3lit air and 1 cartridge of sorb material.
Start off by booking a try dive with an instructor. Do an Open Water + Advanced course on the machine. This will make it possible for you to get another certificate (yeeeeaahh!) and also being able to rent a machine to begin with.