Though 床下【ゆかした】 stands as a noun by itself, it literally means "under-floor". Therefore, 床下30センチ always means "-30cm from the floor level". 床 in this phrase is understood as the floorboards you live upon. If you want to tell you have 30cm deep water in the space between the base and the floorboards, you have to say 床下に30センチ (so that you grammatically treat 床下 as a noun).
Similarly, 床上【ゆかうえ】 is used for "above the floor level" as well.
Or more generally, there are lots of words with ～上/下 that means "(distance) above/below X".
- 桁上【けたうえ】/桁下【けたした】 above/below the girder (see the pic)
- 股上【またがみ】/股下【またした】 (of bottom wear) rise / below the crotch
- 地上【ちじょう】/地下【ちか】 above/below ground
- 氷点下【ひょうてんか】 below freezing (= minus degree Celsius)
"床下 + number" is possibly open to two ways of interpretation (see the comments)... But at least I've found a J-E dictionary entry that says:
The flood waters rose to within ten centimeters of the floor of his house.