Fires cannot be lit on water.
Fires can be lit on ice!
Fires on deep water immediately melt themselves a hole in the ice and sink! This is sensible!
Fires on shallow water melt themselves a hole in the ice, and go 'oh look, I am now an Item In Shallow Water! I will keep burning and melt a large area around me'.
Presumably this is of benefit to anyone who has complained about their ship getting icebound, so maybe it's working as intended for those people fortunate enough to have their docks in shallow water. But I am petty and confused about the large puddle that appeared when I just wanted to do some ice fishing, so I made a post about it.
(if you stand on ice over deep water which then melts when you light a fire in shallow water, you end up swimming even if swimming is not turned on, have fun, don't murder anyone xx).