"Oxygen tank" complicates this answer. Oxygen breathed at depth can have some very unpleasant symptoms. Think of the O2 as the paramedics do. Oxygen is a drug, and you can overdose by taking a lot at once or a little for long periods of time. The deeper you go, the higher the partial pressure of oxygen, the greater the dose your body takes on.
Depending on how long you stay, you can dive to 20 - 50 feet of sea water. However, you won't have but a few short minutes at 50 fsw, before you suffer from CNS Oxygen Toxicity.
However, if you were asking about an "average" Scuba diver, breathing compressed AIR... The recreational limit is typically 130 fsw. This limit is in place due to the effects of nitrogen narcosis (AKA "rapture of the deep") and once again, partial pressure (of NITROGEN this time) is to blame.
Navy Deep Sea Divers are qualified to 190 fsw, but these depths are usually considered "exceptional exposure" dives. They are most often performed while wearing a hard hat diving rig, with surface supplied air, and a support staff tending topside.
swimming you have to go in a pool where as diving you dive into water from a height
I would say scuba diving. You can get the bends, or nitrogen narcosis. There is also the factor of running out oxygen during a dive.
the air inside you
To jump headfirst in to water is to dive.
The board is called a diving board.
when you dive into the water going forward. usually touching your toes and then diving in.
They do not dive deep enough.
Its farther from the surface where the sun reflects.
Yes, it is possible for a human being to dive to 400 feet with the use of specialized equipment such as mixed gases and technical diving techniques. However, diving to such depths requires extensive training, experience, and equipment to manage the associated risks of deep diving, such as nitrogen narcosis and decompression sickness.
A wet suit that has been designed for the temperature of the water you plan to dive in.
Crawl diving.
A dive shop sells all the required components for one to go underwater diving. They sell wetsuits, oxygen tanks, masks, fins, snorkels and accessories.