The player can place boats underwater while looking up

Is there any indication (on the WIki or elsewhere) where this is not the intended behavior?

With the advent of the Turtle Shell helm (mixed with Respiration and Aqua Affiinity) I find myself doing a lot of diving, and I often break and collect my boat as I might come up a long way from when I began my dive. This being so, the ability to place a boat on returning to the surface is quite useful.

I agree with your bug report MC-128895, as it doesn't make sense of the map to need your hands to hold it on land and not underwater, but this seems like the intended behavior.

Is there any indication (on the WIki or elsewhere) where this is not the intended behavior?

No statement by the devs that it's neither intended nor unintended, so it's a valid report. Also note that the wiki is not an official source.