This is inaccurate; water content is one of the easiest things to measure with near-infrared spectroscopy, and NIR has enough penetrance to be useful even for fruits with fairly tough skins (eg avocados, see last link below)
There are multiple commercial manufacturers selling NIR sensors for fruit quality control:
http://www.ndc.com/en/Products/On-Line-NIR-Gauges.aspx
Research articles: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673116/ http://www.cropj.com/miraee_4_3_2010_175_179.pdf http://www.brimrose.com/pdfandwordfiles/I_GRASA-CSIC.pdf http://cdn.intechopen.com/pdfs-wm/36050.pdf (avocados)