After finding I had the exact same issue on my ipad, a quick Google brought me to your post. Like you l also hope they find a fix for Apple devices before the start of next season. I had 2 of the original versions. The user interface is just what I wanted and subsequently purchased 5 regions (although the later versions are from WaterNav). When I click on the icon I just get a notification saying "This App is no longer available in the App Store" and won't open which is strange.
Yes, I also have a much dog-eared selection of Nicholsons guides but I much prefer using a combination of "Canalplan" for route planning and WaterNav for local information whilst cruising. These have become my preferred method of finding my way about the system in recent years. If needs must then I could always use "Open Canal Map" that I check for stoppages and gps etc. but I still like the WaterNav Interface.
Please post again should you find a solution