Scubapro defines cold water as 40 degrees or below. Water warmer than that should pose no problems for a MK 25. SP developed the AF (anti-freeze) kit for the MK 25 in cold water and any new MK 25 will come equipped with the kit anyway. If the reg is old stock (priro to June) it may not be fully AF equipped, but the dealer should install it for you for free before the sale.
Scubapro's latest cold water advice for the Mk 25 is to make sure they have the AF kit installed. This includes a composite piston with a mirror polished stem, and a nifty little teflon coated rubber boot on the stem that essentially acts like a deice boot on an aircraft wing. They also suggest reducing the IP to 125 and ensuring the adjustment knob on the second stage is set to prevent any freeflow.
With low pressure steel tanks you should not have any problems in water 40 degrees or colder unless you are really an air hog. I am not yet completely convinced about the effectiveness of the TIS/AF equipped regs in cold water with high pressure tanks. The adibatic cooling demands are, in my opinon and experience, more than the Mk 25 can handle with or with out the AF kit if you get into a high air demand situation. The Mk 25 just has an incredible flow rate of 300 scfm and can get itself in trouble if you try to use all that flow rate in cold water.
In water over 40 degrees, I have no reservations about using the MK 25 whatsoever. You should have no problems with your intended diving.
