What's wrong with Google docs or any of the myriad online whiteboard apps?
I’m also not convinced that there is a tool that when mastered can provide the same fluidity, but I can claim that, and to the contrary I’d love a recommendation.
It’s definitely not the same as drawing on a whiteboard, but I’ve found it useful for explaining abstractions.
(That said, if whiteboards are important to a team, I would imagine there's software for sharing tablet drawing. I've heard there is. I just haven't investigated.)