I have always used distilled water for my coolant changes for the envoy, but after researching a little it appears the DexCool is more tolerant to hard water so tap water apparently is ok....which is a good thing.
The thing is, if the vehicle wasn't built around the heater core I would feel better but that's just one part of the equation. I have had DexCool in this motor for 11 years and never a problem..knock on wood. Keep up on the service and you should be fine.
For reference, this is from the owners manual:
Engine Coolant
The cooling system in your vehicle is filled with
DEX-COOL® engine coolant. This coolant is designed
to remain in your vehicle for five years or 150,000 miles
(240 000 km), whichever occurs first, if you add only
DEX-COOL® extended life coolant.
The following explains your cooling system and how to
add coolant when it is low. If you have a problem
with engine overheating, see Engine Overheating on
page 5-26.
A 50/50 mixture of clean, drinkable water and
DEX-COOL® coolant will:
• Give freezing protection down to −34°F (−37°C).
• Give boiling protection up to 265°F (129°C).
• Protect against rust and corrosion.
• Help keep the proper engine temperature.
• Let the warning lights and gages work as
they should.
Notice: Using coolant other than DEX-COOL® may
cause premature engine, heater core or radiator
corrosion. In addition, the engine coolant may
require changing sooner, at 30,000 miles (50 000 km)
or 24 months, whichever occurs first. Any repairs
would not be covered by your warranty. Always use
DEX-COOL® (silicate-free) coolant in your vehicle.
This kind of puts the ball in your court if you fill it with green and have issues.