I actually shoot above water as well.

I understand the aspects of the overlap, however, if we throw in the Tokina 16-50, that pretty much takes care of the lower angles of the Canon (and from the tests I've read it's supposed to be sharp at 16 mm, although I may have to take the aperture down to f/4, which is still equivalent to what the Canon gives).
Do I need the 10 mm / 100+ angle that the Canon gives on LAND? Not sure...
Will I be switching between the Canon and the Tokina under water? Not sure here either. I mostly want it for wreck photography in Sweden where darkness and sometimes murky water leads me to believe the Tokina will be on the camera most of the time. I also like the effect that FE lenses give when it comes to wreck photos.
Out of curiosity, are there any samples of shots with the Tokina at 17 mm, preferably of wrecks? And does anyone have sample shots of it on land, again at 17 mm?
I'd say that the 10-12-14 range is what gets most usage on a wide rectilinear zoom. The 10-17 and the 10-22 do overlap here, but sometimes you really need those straight lines (like on a wreck). You can off course straighten a fisheye out in software, but personally I very much preffer to see the final image in the viewfinder. Then, perhaps, you go on a holiday to warm, clear, blue waters and wants to shoot a reef shark or two? The 10-22 will be very useful. The 10-17, in non-baited situations, will be too wide, even at the narrow end. And a midrange zoom will not be wide enough for that lucky close pass. You want that 75-100° FoV. But in the Baltic Sea, the 10-17 will most certainly be on your camera on 9 dives out of 10. So the 10-17 certainly makes more sense initially. But you really should have both
and you already own the 10-22...
The 16-50 is unproven, both on land and UW. And even if it proves to be very sharp on land, I'd be careful in assuming it will automatically be great for UW-use. It's not always the case (some Nikon-glass springs into mind). A lot of things come into play when you put the lens behind a piece of plastic that acts as a second lens and then try to shoot through water - such as finding an appropriate port, the right extension to have it properly alligned and most probably what dioptre to use because it has a CFD j u s t a liitle too far away. If you don't get it all right, superior sharpness on land means nothing. It's not like the 10-22 with it's very good CFD and several 'tested & approved' set-ups. I'm sure everything will be fine and dandy, but even when sorted out, it will not do the same job as the 10-22.
Have you considered getting a cheaper mid-range zoom to be able to keep the 10-22? The Sigma 18-50/2.8 has gotten good reviews and I believe it is a bit cheaper than the Tokina. It is also a 'proven' UW scout-lens.
Anyway, it's your call.
Good luck and good night - putting a 5-year old to bed. I'll probably fall asleep too 
Tack för mig.
/christian