Your dive will automatically start when you are submerged 1.20-1.50 metres and will end when you surface.
When the watch auto-starts a dive, it will default to the last dive type you recorded — so if the last dive you recorded was a multiple-gas dive, it will start a multiple-gas dive.
Go to the predive check page, and look for the green checkmark to ensure your entry point is recorded. Dives starting from the watch face may not record your starting GPS point.