Is it... not the case that any humanoid who can't breathe water, regardless of how long they were or weren't holding their breath before then, would die (presumably with associated cardiac arrest) exactly 18 seconds after they started inhaling water?

Because if so, Creation is confused!  Creation is pretty sure that's how primordial inevitables imagined biology ought to work, albeit possibly without ever having actually seen a humanoid.  If Merrin's sheer presence and force of belief is mandating Reality to have more complex rules than that, in this faraway place where the primordial inevitables may well be paying less close attention, she needs to go explain on her own how she thinks biology ought to work.

Is it... not the case that any humanoid who can't breathe water, regardless of how long they were or weren't holding their breath before then, would die (presumably with associated cardiac arrest) exactly 18 seconds after they started inhaling water?

Because if not, Creation is confused!  Creation is pretty sure that's how primordial inevitables imagined biology ought to work, albeit possibly without ever having actually seen a humanoid.  If Merrin's sheer presence and force of belief is mandating Reality to have more complex rules than that, in this faraway place where the primordial inevitables may well be paying less close attention, she needs to go explain on her own how she thinks biology ought to work.