What you're seeing in the heel stack is leather, so maybe it did swell with the water. Maybe it's not all the way dry, either--perhaps the outside dried and returned to size (or shrunk slightly) leaving the middle inside still a little damp (swelled). That might cause a slight gap along the outside edge. And since it's leather, maybe it will settle back once the middle is also 100% dry and you start wearing it again--leather conforms, so... I'd give it another 24 hours to dry and see what happens.
The good news is that since the heel is nailed in place, you might (probably) be just fine. If I were you I'd give Alden a call. Here is the contact info for the different locations, who have been helpful whenever I've had questions (especially the San Francisco one). And I'd be sure to also ask them whether you need to condition the heel stack and if so with what--since the leather got water logged and then dried out I would think it needs conditioning, but I could be wrong.
Alden San Francisco,
https://www.aldenshop.com/" onclick="window.open(this.href);return false;, (800) 273-7463,
online@aldenshoe.com
Alden New York,
https://aldenmadison.com/" onclick="window.open(this.href);return false;, 212-687-3023,
https://aldenmadison.com/contact/" onclick="window.open(this.href);return false;
Alden Washington, DC, (202) 347-2308
Alden of Carmel,
http://www.alden-of-carmel.com/" onclick="window.open(this.href);return false;,
question@alden-of-carmel.com
cheers 