The series of reports "Annual Statistics of the City of Tokyo" (東京市統計年表) gives insight into the demographics of Tokyo from 1905 until the 1940s. In particular, for each ward of Tokyo (they weren't special wards then), you can look up its population at a given time. Fortunately, a large part of the series is made available online free of charge.
In 1908, there were just 15 wards. Surprisingly, the table (on the lower right here) for that year includes a 16th row with the label 水面 which I couldn't make sense of at all (I first thought maybe it was the name of a ward that only existed for a short time).
Originally, the series was published in English, this only changed after issue 25 from 1929, when the series switched entirely to Japanese. Since those English issues cover the year 1908 already, naturally, it's possible to see how 水面 was translated.
Indeed, in the issue from 1929, e.g., the population is split into "Lands" and "Waters". This distinction may seem a little unnecessary since the "Waters" population is always stated as "-" in that table, but as you can see on the next page, in later years they did indeed collect corresponding data for all districts.
So, my actual question is: What is the meaning of 水面 here? What is "waters population"? Please note that on the next page that I mentioned earlier, which gives data from 1920, you can see that many wards of Tokyo had such "waters" population, not just those that were by the Ocean, including e.g. 牛込区 (Ushigome-ku).