Yes it would be nice... or that the ui would auto-hide if not used, like healthbar is hidden when out of combat and so on.

I did the same thing last night while trying to take some screenshots. I looked all over and ended up having to disable every single option to hide everything, the re-enabling them when I went back to playing. Really takes away from the experience when I have to spend a few minutes doing that for one screenshot. Maybe that function with be added later, but aside from that I am enjoying the game quite a bit so far.

I wish GOG would add a screenshot function to Galaxy also, as I had to use Fraps instead.

Hello! I use controller so :P

GoG should add PCShare feature. :D

healthbar is hiding when fully healed and out of combat , but a hide ui key would be nice instead of tunring off everything all teh time u take a screenshot

Please add this into the game. It was in the previous one.

While CDPR takes time to implement an hide hud key (please do it!!!) there are some mods that allows you to achieve what you want to take awesome screenshots:

Immersive HUD by Gopher (it works but Gopher suggest to use Friendly Hud)
Friendly Hud by wghost81 (It seems to be the most complete mod to do this)

Two more mods for some extra effect for your screenshots:
The Witcher 3 Visual Immersion Overhaul by Lunayah
Immersive Cam by KCS

Have a good screen hunting!

[EDIT]
Little update: I've got a hud-less screenshot by pressing ESC to get into the game menu and pressing it again to close the menu and getting back into the gameplay. There's a very small gap between the menu closing and the HUD loading that allows you to capture screenshots without the HUD (it's a very small gap, if you can time your finger well, it works!)

Little late to post this, but figured I would for anyone else wondering.

I'm playing on PC, and when I press the "home" key it disables the HUD entirely, and pressing it again toggles it back on.

Better late than never: Pos1 is the key.

press the 'home' button