Meaning of 水面 in reference to population


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](http://kindai.ndl.go.jp/info:ndljp/pid/1461323/62)) 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](http://kindai.ndl.go.jp/info:ndljp/pid/1448372/157), 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](http://kindai.ndl.go.jp/info:ndljp/pid/1448372/158) 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).




The linked English version has an answer:

[![clip](https://i.stack.imgur.com/X8Ish.png)](https://i.stack.imgur.com/X8Ish.png)

> 本表中ニ水面トアルハ船舶等ニ依ル水上現住者ナリ 合計ハ水面ノ數ヲ含ム  
>  Population on "Waters" given above are those living in the ships or vessels
> in the port or such like places.

Apparently this category was used only in the statistics of the year 1908 (see
[this
PDF](http://www.soumu.metro.tokyo.jp/01soumu/archives/0714ku_jinkou.pdf)).




The linked English version has an answer:

[![clip](https://i.stack.imgur.com/X8Ish.png)](https://i.stack.imgur.com/X8Ish.png)

> 本表中ニ水面トアルハ船舶等ニ依ル水上現住者ナリ 合計ハ水面ノ數ヲ含ム  
>  Population on "Waters" given above are those living in the ships or vessels
> in the port or such like places.

Apparently this category was used only in the statistics of the year 1908 (see
[this
PDF](http://www.soumu.metro.tokyo.jp/01soumu/archives/0714ku_jinkou.pdf)).

