The biggest issue was that the source of the liquid wasn't established and that it might have not been safe to drink, potentially causing bigger problems than Luca immediately faced. He did try to drink a bit of the fluid, and reported that it has an odd taste, so it was not his drinking water. They opted for immediate retreat back to the Quest airlock and proceed with emergency repressurisation of the airlock, so he could take the helmet off as soon as possible with the help of other of his ISS crewmates. It still took long and agonizing minutes to complete all these procedures, as Luca described in his EVA 23 blog post:
I move for what seems like an eternity (but I know it’s just a few
minutes). Finally, with a huge sense of relief, I peer through the
curtain of water before my eyes and make out the thermal cover of the
airlock: just a little further, and I’ll be safe.
I try to move as little as possible to avoid moving the water inside
my helmet. I keep giving information on my health, saying that I’m ok
and that repressurization can continue. Now that we are
repressurizing, I know that if the water does overwhelm me I can
always open the helmet.
This rushed ingress turned out to have been the better option, and Luca could still have drunk some of the liquid if he couldn't breathe any longer due to it filling his EVA helmet. The exact cause of the leak of what was later determined to have been the Portable Life Support Unit (PLSS) cooling liquid has not yet been determined, except that it was reaching the helmet through the T2 helmet port so it originated from PLSS, but the general consensus is that it's better that he didn't drink the liquid up, unless he couldn't help it any longer and was simply left with no other option.
Luca figured he could alternatively try and perform a controlled depressurisation of his helmet, but that would be a risky procedure:
The only idea I can think of is to open the safety valve by my left
ear: if I create controlled depressurisation, I should manage to let
out some of the water, at least until it freezes through sublimation,
which would stop the flow. But making a ‘hole’ in my spacesuit really
would be a last resort.
For now, since the root cause for liquids filling up Luca's helmet wasn't yet established precisely, they're settling with temporary fixes to prevent further such incidents by adding absorbent pads to the Extravehicular Mobility Units (EMU) and equip the helmets with a snorkel device.
Some relevant links:
Update: NASA made the full report on the Mishap Investigation Board investigating US EVA 23 mishap and its root causes available to the public in the International Space Station (ISS) EVA Suit Water Intrusion, IRIS Case Number S–2013–199–00005 (PDF):
In summary, the causes for this mishap evolved from (1) inorganic
materials causing blockage of the drum holes in the EMU water
separator resulting in water spilling into the vent loop; (2) the NASA
team’s lack of knowledge regarding this particular failure mode; and
(3) misdiagnosis of this suit failure when it initially occurred on
The source of the inorganic materials blocking the water separator
drum holes had not been experienced during an EVA before and is still
undergoing a concurrent investigation. The results of this
investigation will ultimately lead to resolution of this issue;
however, since the concurrent investigation into the source of the
debris is expected to continue for many months, the MIB does not yet
have the required data to determine the root causes of the
contamination source, which must ultimately be determined to prevent
Related article on Space.com: Spacesuit Leak That Nearly Drowned Astronaut Could Have Been Avoided