I haven’t followed all the discussion on talk-ca and talk-us about how to tag the North American Great Lakes, but when looking into this problem:
I did run across mention of this consensus:
and so I’ve started to tag the edges of Lake Ontario as “natural=coastline” to bring Ontario into consistency with the other Great Lakes.
This may violate someone else’s notion of what “natural=coastline” is meant for, but until such time as there is another method brought into practical use for rending the Great Lakes a) at a reasonable zoom level (eg, they are big enough that they should show at the default zoomlevel of 5) and b) consistently (eg, showing all 5 lakes, instead of just 4 of the lakes at zoom 5), this seems to me a reasonable compromise.
If you disagree, please implement a different fix.