Could be two senses.

also, apparently the japanese follow the scottish in not using the 'e' for their whiskies, to which this would normally be applied

@@ -17 +17 @@
-<gloss>cutting an alcoholic beverage (usu. whiskey or shōchū)</gloss>
+<gloss>cutting an alcoholic beverage (usu. whisky or shōchū)</gloss>

koj
アルコール度を弱めるためウィスキーや焼酎などに水を加えること。また、その飲物。

https://www.dictionary.com/browse/cut

not sure that the adj-no is even needed as a separate sense.  pretty obvious from context.  and when i hear "watered", i think "watered plants", not "watered whisky".

also, don't you "cut" whisky?  to me, "watering" alcohol is what you'd say about bar managers doing it to save money, not what you'd do to make the drink more palatable or get you less drunk

@@ -16,3 +15,0 @@
-<gloss>alcohol (usu. whiskey or shōchū) diluted with water</gloss>
-</sense>
-<sense>
@@ -20 +17,5 @@
-<gloss>watered</gloss>
+<gloss>cutting an alcoholic beverage (usu. whiskey or shōchū)</gloss>
+<gloss>diluting an alcoholic beverage</gloss>
+<gloss>cut drink</gloss>
+<gloss>diluted drink</gloss>
+<gloss>watered-down drink</gloss>

Tanaka: 水割りをもう一杯ください。 Another Scotch and water, please.

The 2 Tanaka sentences and the GG5 etc. entries show it's often used for "whisky and water". GG5 is effectively saying that 水割り1 杯 is an abbreviation of "ウイスキーの水割り1 杯".
Perhaps the senses should be reversed and the alcohol one marked as an abbreviation.

Isn't sense 1 better translated as "diluting alcohol with water"? I'm not sure but I feel the usage is ”水割りで" rather than "水
割り一つ", in general