A mage is traveling by ship, and wants to keep cargo on the deck from being washed away by waves. So inscribes a circle on the (hopefully water-proof) deck around the cargo, and casts a Circle duration spell "Ward against seawater". (variation on "Break the oncoming wave" ReAq from the main book)
No (natural) seawater can cross this ward. What happens if the ship sinks (without capsizing, nor leaning much)?
Would there be a column of air from the sea bottom to the sea surface? At least until water soaks through the decking?
Your sinking ship would be similar, I believe. Would the circle apply against the seawater washing overhead to create a cylinder of air, or would it just protect what's inside the circle, creating a bubble?
I think the best answer is that magic should be erratic and unpredictable, and both interpretations are sufficiently meaningful to toss a coin each time it comes up.
OTOH I believe the point is moot. The cargo would dislocate and damage the circle and thus break the spell long before the distinction matters.
If the ship sinks, we have to assume the hull was breached. In this case, while a circle on the upper deck might prevent seawater from getting in from the upper deck, it wouldn't prevent seawater from entering through the pierced hull. Nor would it prevent air from escaping. As the ship sinks, no air bubble would remain, and seawater would replace the air by coming in from below. There would be a very short lived period of time where there are water walls arround the ship while it sinks.
You are right that it would not prevent the air from escaping, but what happens? Does the water prevent the air from escaping? Or do we get a vacuum bubble?
I do not think demons can enter a warding circle through the floor, nor do I think a mole can tunnel into a ward against animals. Allowing it would be as bonkers as letting a hippogrif fly into your Aegis and attack from above. It should be the same as for seawater.
Admittedly, I do like the kind of restrictions that were common in 3ed but not warranted in the 4/5ed guideline system, where such a water ward would be broken if submerged in water. I think there is a canon (Ind Target) spell where this legacy restriction has survived.
My thought is that seawater would collect on the surrounding deck, putting weight on it which is not on the protected circle, and eventually the difference in sheer forces would split the deck, such that teh circle is now a raft above the waves which is protected from the seas while the remainder of the ship is at the bottom of the ocean, save for any bits and pieces which broke off and are able to float.
I'm not sure why the air would escape, I don't believe there is a concept of vacuum so if the water cannot enter...
in any case it seems the key question is how the ship's deck breaks- the circle either becomes a raft protected from water or detritus of a shattered circle.
I know. My modern-era thinking often gets in the way of the Mythic Europe paradigm, which is why I end up asking questions like this on the forums, in order to get my brain in the right frame of thinking.
the nature of air is to ride to its own level- so he air tries to rise but the water can't get in, so the deck is simply pushed up, creating the forces described that either shatter the deck in a circle or shatter the deck across the circle.