The Holodecks and Holosuites of Star Trek fame have a number of security measures designed to ensure that no part of the simulated environment can harm the user. Assuming there is no malfunction and no override, no harm should come to a user of the holodeck (and clearly that would never happen).
However, in Star Trek: Generations, we see that both Commander Worf and Doctor Beverly Crusher are fully capable of being submerged in Holodeck water.
Water can be a dangerous substance - if you are underwater and swallow too suddenly, you could choke. If you choke on water or cannot breathe under it, you'll drown.
The Holodeck is supposed to keep the user safe from physical harm, such as from blades, bullets, and even holographic punches, but it uses simulated molecules - so all these objects are still real. So it should still be able to block the user's airway.
Is there some way in which the Holodeck can prevent this? Can it actually stop the user from drowning?