One of nature's cruel tricks is that Carbon Monoxide is far better at bonding with Haemoglobin in Red Blood Cells than Oxygen is.
Always have cylinders filled at reputable filling stations, check compressor s regularly and have properly located intake pipes.
Because of the increased density of water sound travels approximately 4 times faster through water than through the air.
Every heard someone make a banging sound while diving? It almost always sounds like it's coming from above as the sound hits both ears at the same time causing you to instinctively look upwards.
Slack tide or 'the still of the tides' is when there is no movement in the tidal streams; it is neither ebbing or flooding. This can be a great time to dive as there is less water movement.
Hight tides are seen as preferable as they can make entries and exits simpler from shore and bring upwellings of clearer water. Perfect for budding photographers.
Although the details for PADI Speciality Diver or Specialty Instructor ratings aren't listed in the PADI Instructor Manual, the fundamental information for these courses can be found in Key Standards found between Training Standards and Paperwork and Admin Procedures.