Where is that number coming from? My quick search suggests UV penetration is not that severe (although still not to be ignored).
For example, from "Penetration of solar UVB radiation in shallow tropical waters and its potential biological effects on coral reefs; results from the central Indian Ocean and Andaman Sea" by Richard P. Dunne and Barbara E. Brown -
https://www.int-res.com/articles/meps/144/m144p109.pdf
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The three locations have decreasing water clarity. PAR is essentially visible light.
According to their measurement, even in the clearest water, 50% of the UV A+B is absorbed by around 2 meters. Unless I'm reading this wrong, which is certainly possible.