Good news everybody! Asobo have added several hundred water runways to the game! After I updated my Little NavMap database it shows 701 water runways. Unfortunately though it seems the overwhelming majority of them lack parking spots so you start with everything turned on at the actual water runway instead of cold and dark. Almost all of them are contained within the US as well with a handful in Canada and Sri Lanka. Still better than nothing eh? Just took off from Commodore Center just north of San Francisco in the Cessna 172 amphib!
Screenshot of LNM with all the water runways in USA+Canada selected:
Edit: Just noticed that was only 250 odd selected. There’s a fair few more including more in Canada and some dotted around Europe (mostly France), a couple in New Zealand and a couple in Fiji