I have a copy of Principles of underwater sound by Urick, 3rd edition. In the chapter on The noise background of the sea: ambient noise level, on pages 215/216, it says
"The analysis of data in the frequency band 20 to 100 Hz revealed periodicity of 12 and 24 hours, with a maximum of noise at noon and midnight, local time, at all locations, independent of longitude. Although the change in level was small (normally 1.5 to 5 dB), it was undoubtedly real, but the origins of these changes in noise, which are apparently free from biological activity and synchronized to the mean solar day at diverse locations, remains a mystery."
The book was published in 1967, and my copy printed in 1983. I'd be fascinated to find out if there has been any new research into this. I did a little googling but couldn't find any references to this issue.