Problem found like this:

• Seed: -204599536
• Show coordinates: ON (shouldn't matter but I always enable this)

I tried creating this world five times, with different combinations of game modes (survival or creative), and regular or Education Edition.

Each time, I spawned underwater, in a small pond approximately 3x3x3 in volume. One cannot immediately escape the pond without digging a block from the side to create a shoreline to step onto.

Quite a panicky situation for a new player! Fortunately I'm experienced.