Quite a number of sources disagree with the absorption spectrum you found, placing the minimum at around 500 nm.
https://psu.pb.unizin.org/eshanichemistry110/back-matter/appendix-e-water-properties/
Optimal light transmission would occur in the 425nm range but he is asking for a device that can penetrate 20m and I assume he does not want a 100w strobe light so recommended using the top end of the UVA range for invisible light and it would also reduce interference from other visible light sources.
The question was about ultrasonic transducers.
Anyway, it is interesting that there is some disagreement in the literature, and that probably comes down to the techniques used for the measurement.
Have fun!
Indeed, and I offered him a possibly easier and more effective solution to problem he was inquiring about. Is that not what these forums are for?
His question was about how to send a modulated signal to a lobster trap at a depth of 20m to inflate a buoy to surface the trap on command. Transducers with a good water interface are expensive, finicky, and power hungry, while a 100w 395nm UVA led cost $35 and can be lensed for another $5 without any of the technical issues associated and provide communications out to 75m.
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