5ft. step...but what about flipper and flap?

[Stone the Crows]

Can you make a 5ft. step in water [flipper] or air [flap]?

[Dawn Reed-Burton]

I'm pretty sure you can't do that in water, because water is classified as difficult terrain & you can not take a 5 ft. shift in difficult terrain without a feat or special class feature.

[Erik Freund]

Flying has its own restrictions around "must keep moving" unless you can succeed a "hover" check. On rounds you take a move action, you cannot take a 5ft-step.

However, if you are "hovering", then there is nothing stopping you from taking a 5ft-step laterally or downward. You could not take one upward, however, as that explicitly takes 2 squares of movement.

[Erik Freund]

Dawn Reed-Burton wrote:

I'm pretty sure you can't do that in water, because water is classified as difficult terrain & you can not take a 5 ft. shift in difficult terrain without a feat or special class feature.

Specifically, if you have a Swim speed, then you can take a 5-ft step. Without a Swim speed, then Dawn is correct.

[Fionnabhair]

If you have a fly speed or a swim speed, I believe it's possible to take a 5-foot step. Source (last line).