The 850, 870 and 950 are all very similar.
6 months ago, I selected the sd850 for my wife and
I went through this decision process again just a month ago
when picking a new p&s camera for myself.
The 870 has a wider field of view. 28 vs 35mm.
The 850 and 950 have an optical view finder, the 870 does not.
The 850 has "teeth" on the selector wheel that can use a gearing mechanism
in the housing. The 870 and 850 use a pressure/contact fitting in the housing
to rotate the selector wheel.
The 950 is 12 mp, while the 850 and 870 are 8 mp.
The 850 & 870 have a playback/review mode that requires moving the selector wheel.
On the 950, you can jump in/out of playback/review mode by a new button that
isn't on the 850 or 870. This makes it easier to delete unwanted photos
on the 950.
The CHDK firmware which adds all kinds of features including a battery meter
and being able to shoot RAW, is currently only available on the sd850.
Pick your favorite features, and you will quickly narrow down your selection.
For me, 8 vs 12 mp didn't matter.
I wanted an optical view finder for land use, so that threw out the 870 even though
I thought having a wider angle would have been nice.
While I preferred and really wanted the playback button to allow quick
tossing of bad images while shooting (including underwater) I ended up opting
for the sd850 because it has CHDK available for it right now.
In my opinion, as far as housings go, I would use the Canon housing
over the Ikelite housing because Ikelite
didn't put in a button for using the <print> button. This is mandatory in my view
because you can re-assign this button to custom white balance. This allows
you to set white balance with a single button click. Without this, it is about
11 button clicks on the sd850 to go through all the menus to do a manual
white balance.
There is no comparison in color quality between an image taken with manual/custom
white balance and one taken using the canon underwater scene mode.
I found that the canon underwater mode wasn't very good.
So setting white balance needs to be easy since it is done quite often.
(unless shooting RAW, which can only be done with CHDK on the sd850)
--- bill