If you mean specifically about the spacing between the number, 40, and the units of measure, mm, then you are correct strictly speaking - there is a "Style Guide" for SI units:

https://english.stackexchange.com/questions/2794/punctuation-with-units

which recommends having a space between the 40 and the mm as so: 40 mm.

If you mean "is it correct English" to say: "40 millimetres of water" - then yes, that's a generally correct syntax: for example in a cooking recipe, you might read something like "add 500 mg of butter" or "measure 100 ml of milk" etc.

If you mean "is it correct to measure water amount (logically, a volume) using a single linear dimension": then yes, this is referring to water height as measured by a pluviometer (or Rain gauge in english language).