Labels should intuitively “connect” to their respective object on a map so that map reader could “feel” the stronger connection between the label and waterbody (lake natural=water or reservoir landuse=reservoir). When calculating places for labels in multi-scale (or vario-scale) map, we can distinguish three types of label placement:

1. Multiple large labels - this is used in large scales where waterbody is so large that a very large label could fit. Usually at that point waterbody is so large that only part of it will be visible to map reader. This means multiple labels could(should?) be placed, trying to calculate the average size of a map view - we would like one and only one label to be visible at one point and as label positions have to be calculated beforehand it is impossible to make precise calculations as maps would be viewed on different devices with different size of map canvas. (There is also an interesting question if such labels should be horizontal or not, if not - how should they be positioned?)
2. Curved labels - middle scale labels where label can be curved according to the geometry of a waterbody.
3. Simple labels - small scale labels, where it is no longer possible to have curved labels (as they no longer fit into waterbody) and only simple straight line labels are possible.

Note that type of label depends on a particular scale+waterbody. That is on one scale you could have some waterbodies with large labels, some with curved and some with simple labels. As can be seen here:

Topographic map of Lithuania

Here you can see a large curved label for lake Želva, smaller curved labels for lakes Gilužis and Trinktinis, and a simple label for lake Lenktinis.

The most interesting are curved labels. While calculating approximate medial axis could look like a good way to get a curve to draw a label on, it has one major disadvantage - you cannot get information on how large your text could be (and what letter spacing you could use). One might think that adding buffer to the line would indicate the possible size of a type, you will still have problems with irregular shape waterbodies, where label should be placed on a side where waterbody is “large enough” for a label (think of a waterbody with a shape of a prolonged triangle).

OpenTopoMap has used a very interesting algorithm to divide a waterbody to squares and then get a curve for a label. You can read about it on Github. This solution can be extended by iterating through different sizes of squares: starting with larger ones - trying to fit large type and then decreasing the size of a square thus trying to fit smaller type. The result of such calculation is seen in the picture above.

Such solution fits most of waterbodies. Even more interesting solutions are required for very irregular shaped waterbodies, say lakes with a shape of U, E etc. Such waterbodies should probable require more than one label placed even in one view as connection between different parts could not be obvious. Note that there is no single accepted cartographic convention if one object should have one label, or it could have more that one label.