How many natural=water are in OSM?

You have actually asked two unrelated questions. The second question about the total number of water bodies in OSM is still open. So, let me try. Assume, we understand the same thing under the notion "water body", namely - a simple area defined by one and only one outer/enclosing border polygon and a zero or an arbitrary number of inner/excluding/hole border polygons (so, the water area is between these two border classes). Then, the water bodies in the same water class (like rivers, channels, lakes and so on) are isolated/unrelated objects (even if one is in a hole of another one, no common border sections exist ...) and the numbers bodies per classes are: 1,509 in the sea class (based on coastline data as an inverse of the land bodies), 5,630,410 in the lake class (inclusive basin, reservoir and intermittent water classes as well), 307,659 in the river class and 4,265 in the channel class. In total, 5,943,843 water bodies.
Some related notes. 1.These numbers are constantly changing. 2.Some water bodies are huge and extremely complex. Example gratia, the Global Ocean contains over 488,080 holes, the Amazon, Mississippi ... river system is really complex. 3.If we merge the water classes, the number of water bodies will decrease but the objects will become even larger and more complex (many river systems with the Global Ocean will create a new huge/monster water body).