Might work for technical divers but regular leisure divers use compressed air which contains nitrogen anyways. If you prebreathe pure oxygen and then begin the

Might work for technical divers but regular leisure divers use compressed air which contains nitrogen anyways. If you prebreathe pure oxygen and then begin the dive using regular air, you will reaccumulate the nitrogen pretty quickly, I think, so you will need to decompress like normal. And you can't dive with pure oxygen because it becomes toxic when going lower than about 6m due to the high partial pressure. Also, you would need to have pure oxygen available and breathe that for maybe an hour before each dive, which would be expensive and annoying. For touristic dives, especially along the shore, it's easier to just plan the dive so you can still see something interesting while waiting for decompression.