Network-wide, and written in the help centre:
When creating your new tag bear in mind that tags …
- must be shorter than 25 characters
must be shorter than 25 characters
- must use the character set
a-z
0-9
+
#
-
.
must use the character set
a-z
0-9
+
#
-
.
Uppercase letters in tags are, unfortunately, not possible because of the restrictions SE places on them. I don’t think that the airport codes here and the few other occasions will be enough to change that policy.
If your question was about costal features, maybe seaside could do the trick?