It’s so nice to be by ourselves.
Does 水入らず give the connotation of “water not being able to enter in” and thus being undisturbed?
I sthe pronunciation みずはいらず or みずいらず? (The sentence pronunciation says the former, but the annotation says the latter.)
Yes, your understanding is correct. 水入らず is a frequent idiom, meaning “water doesn’t enter into a close-knit relationship” and thereby “without disturbance”. 水入らず is mostly to be used between parent-children, spouses, or siblings.
The only correct pronunciation is みずいらず.