In my internal backstory, Pohl did some preparatory Wikipedia research on handshakes beforehand.
Also, if you don’t usually read the Director Commentary (The Transcript button below), make sure you check it during this arc. Today I discuss some additional background work on the sarnothi and their handshake, and a little extra backstory on Pohl. Yay for ancillary content.
Although the premise of "lowering yourself" before a superior does exist in Sarnoth, in a fashion. Culturally, it is rude to float or swim at a higher elevation than a social superior. If traveling together the social lesser does so at a lower elevation, and if addressed or summoned the social lesser must be the one to take initiative to correct any discrepancies in elevation (for example, if a social superior addressed Pohl while Pohl was currently swimming above him, it would be Pohl's social obligation to swim downward and place himself at a lower elevation while addressing his social superior.)
Backstory bonus extra: During his first year or so on the surface, while taking medical school courses, Pohl had to learn not to be very uncomfortable with being taller than some of his instructors. He had a lingering social obligation impulse to "swim lower".