I have scuba dived out of an 18 foot canoe, but you need to be careful on entry or exit unless you use outriggers.
Concerning using scuba out of a kayak, very early on there was room on these kayaks for a single steel 72 tank. I'll see whether I have a photo for you; I did find one, of me with fish I had shot somewhere around 1962, but it only shows the bow of the kayak.
I've enclosed a copy of the September 1965 issue of the Oregon Council of Diving Clubs Newsletter, just for you to enjoy. I was hoping there was something in there about diving with a kayak, but no such luck. However, that front photo shows the Otter Crest area, I believe, and we did take kayaks out there for spearfishing.
SeaRat