Is underwater photography actually possible without a strobe? using the right Camera/Sensor Size?

You can certainly shoot photos or video... they're just going to be really blue unless you stay really shallow.

You might consider black and white... It's a great look and forces you to really work on composition.

Yes it can be done, I wrote a short article that is on the Optical Ocean Sales blog site about it. Of course you can do it, using relatively bright LED lighting but you can't capture action and will have little DOF control.

Bill

One of my friends consistently gets great shots using a canon point and shoot with no flash but uses off camera lighting from a SOLA mounted on the hand grip of her opposite hand.