Saith Bully Putin. But the Sea can't be internal water just of Russia when much of it borders part of what, even Putin admits, is still part of the the Ukraine. International law says the Kerch Strait is a strait and navigation through it must be permitted. So send through a ship named for a Nobel Peace Laureate, laden with media who thinks that prize was deserved, and backed by the real politic strength of a President who knows better. Would Putin arrest the CNN embeds? Would he sink a scow named for Obama? He understands PR to well to do either.
And if Putin wants to sail up Niagara falls he's free to try it, but should be aware our environmentalist lawyers will quick to sue him for any damages done to them. And the locks around the falls are clearly not natural straits.
No, it is an internal water of Ukraine and Russia due to a 2003 treaty. AFAIK, those two are the only ones that can enter without permission.
And when Ukraine threatened to blow up the new bridge, there is a lot of tension now that lead to this.
But in any case, no NATO ships will be going in there.