I will mostly echo what others have said in this thread.
1. Many factors plays a part in this, what size of tank? what pressure rating? your surface air consumption rate? depth? workload? all will be covered in your initial ow training
2. Yes! there are many things to know and hidden dangers to be aware of, even on shallow dives, many potential problems is not directly from the depth you are at, but the fact you are breathing compressed air at a higher pressure than atmospheric pressure, most notably of problems is lung overexpansion injury, also things like entanglement, hypothermia, panic attacks, disorientation because of bad vis and claustrophobic attack. An another factor is as a fresh diver, especially without any training, will be task loaded no mater what, and throwing in something like a metal detector into the mix and you have to deal with things like stress, high air consumption, going low on breathing gas and so forth. Even at a depth of 10 feet the pressure will be around 30% higher then at the surface, meaning the the air you breath at depth will expand 30% when you ascending, the difference between your vital lung capacity (around 4.5 liters) and your total lung capacity (around 6 liters) is around 25%, meaning an uncontrolled ascent, because of a panic attack, out of air situation or other reasons from that depth, have the potential to pop your lungs like two overfilled balloons.
3. Again depends on a lot of factors, that will be covered in your ow training.
4. yes, see answer "2"
5. Yes, if you have a compressor capable of delivering compressed air in the 200bar-300bar range, of breathing quality air, those compressors are expensive, requires regular filter changes, regular air analyzes and maintenance, we are talking 1000s of $ for the compressor, and 100s of $ in maintenance, your run of the mill hardware shop compressor capable of 10-20 bars of non-breathable air won't cut it
