First of all, I don't own a 2610 (yet

)
I don't know whether there will be a "SPIV" of course, but it does seem unlikely to me. I think the 2610 is it.
As for waterproofing, the 2610 is just as waterproof as the SPIII or the GPS V.
quote from the Garmin manual:
Case: Rugged; fully gasketed; IPX-7 waterproof (only with CF slot cover closed. Waterproof rating for StreetPilot unit only, does not include remote control)
The only issue is the touchscreen if you ask me. With heavy wintergloves that could be problem. As far as you need to control it while on the move... From what I have read on several forums, it's not a problem with light or medium heavy gloves.
On my current SPIII, the main thing I use while riding is the (too damn small) zoom in/out buttons. On the 2610 with the much faster screen redraw I'll be using the autozoom I expect. That will eliminate a lot of the need for using zoom buttons.
I have never used any batteries with my SPIII, because I have wired it to the bike and the car. And the batteries wear out way too fast too. But if you want to use your gps for hiking or bicycle riding also, you do have a problem with the 2610 of course.
Cheers,
Michel