https://kotobank.jp/word/磯・礒・石-2006228

礒 is in nikkoku (along with 石, although I'm not sure if that would be helpful to add). Normally we apply [sK] if a form is rare and only appears in this ref. 礒 fits both criteria.
If nikkoku-only forms don't get any counts at all, then we usually don't add them.

Google N-gram Corpus Counts
╭─ーーーーー─┬────────┬───────╮
│ 磯の香り　 │ 90,320 │ 98.7% │
│ 礒の香り　 │    365 │  0.4% │
│ 石の香り　 │    170 │  0.2% │
│ いその香り │    638 │  0.7% │
╰─ーーーーー─┴────────┴───────╯

A more meaningful statistical comparison... いその (Apparently japanese people like the smell of the shore, 磯の香り is a surprisingly common hit).

"correct" kanji
磯の香り	90320	  
磯の香りが	32816	  
磯の香	13757	  
磯の香りと	9330	  
磯の味	8847	  
磯の香りを	8280	  
磯の風味	7918	  
磯の香りがし	4604	  
磯の宿	4046	  
磯の香が	3776	  

"other kanji"
====
礒の	1607	  
礒の香り	365	  
礒の香	132	  
礒の香りが	123	  
礒の上	65	  
礒の浦	54	  
礒の香りを	42	  
礒の香り満載	39	  
礒の上に	36	  
礒の鮑	35	  
礒の浦廻	32

Just ran into a more diverse term for comparison.  The less-common kanji is definitely sprinkled about everywhere いそ is used.  "礒の香り" is still 0.5% (1/2 percent).

For example...
磯釣り	133421	99.0% many news hits 
礒釣り	1295	1.0%  zero news hits

Does not appear in word lookup for 磯 in nikk or daijs (online, at least)
https://kotobank.jp/word/磯-432684

Entry does not exist for this kanji
https://kotobank.jp/word/礒



Example n-grams (all names)
礒	112893	  
礒部	41211	  
礒野	19764	  
礒野佑子	11860	  
礒崎	6888	  
礒貝	6851	  
礒谷	5824	  
礒部選手	5357	  
礒太郎	5355	  
礒部の	5158	  
礒太郎の	4992

Absolutely no other terms in jmdict use 礒 in words. 

礒 not listed under 磯 in daijs. (not in sankoku, but not surprising)

Popularity of 礒 appears to be based on its existence in *names* (same meaning, same readings in names). Very common in names, well represented in news entries.

Is used on twitter occasionally. Without dictionary support, seems best to treat as "incorrect" usage.

@@ -10,0 +11 @@
+<ke_inf>&sK;</ke_inf>