up to her neck vs up to my neck

up to her neck and up to my neck both are adjectives.

Word	Noun	Adjective	Verb	Adverb
up to her neck	No	Yes	No	No
up to my neck	No	Yes	No	No

As adjectives, up to her neck and up to my neck are synonyms defined as:

Other synonyms of up to her neck include neck-deep, up to his neck, up to our necks, up to their necks, up to your neck.

up to her neck (adjective)	up to my neck (adjective)
deeply involved	deeply involved