I had an older housing (poly, similar to O/I's) for about 2 yrs. It had room for a monitor in the back of the case but somehow I never got around to it. But I had sort of an instinctive feel for what I was filming. My buddy otoh, cut the tops off of everything he filmed one trip.
I now have an Amphibico housing, their monitor is a $1400 option so I'm looking for a cheaper alternative, mostly because I would like to film small critters in macro and I can't see well enough through the camera's viewfinder to do so.
Most of the time, you'll likely be filming at a very wide angle setting so having a monitor is less of an issue. I've not found it particularly difficult with either housing I've owned to see the viewfinder at least well enough to frame shots. And I do alot of point & shoot with surprisingly good results.