Mrs H. found these tumblers as a Christmas present for her father, who spent his career at sea, first as a merchant seaman then as a pilot. The advice “do not load above this level” is probably wise, even if you’re not drinking on the deck of a pitching ship.
I can’t really think of a reason why you might fill your whisky tumbler to different levels depending on the season or salinity of surrounding water—except perhaps that if you’re in the tropics then you’re probably on holiday so you can let rip and if it’s winter and you’re in the North Atlantic then the risk of spillage may be higher so caution when filling might be advised!
If you know someone who might appreciate some Plimsoll Line tumblers they can be had from from Nauticalia for £4.99 each or MarineStore for £4.34.
* I did wonder if the energetic Mr Plimsoll then went on to invent the eponymous canvas training shoe, but it turns out that Plimsoll shoes were not officially called that at all. It is a nickname given to them because the horizontal line often found along the edge of the sole reminded people of the Plimsoll Line.