Cheapest way is a set of digital charts on a small tablet. ipad Mini is big enough, you do not need a screen as big as a standard ipad. However, as you say, all the waterproofing cases render the tablet touch screen essentially useless! I have a small dedicated Garmin chartplotter (the GPSMap 620) which is for my yacht but I also take it on the Wayfarer. It is small and 100% waterproof (with no case) so still works when wet despite being a touch screen. Gives your exact position over a Garmin Bluechart and has all the detail of an admiratly chart and more (even aerial photos of harbours, estuaries etc). The small screen matters not as the detail changes as you zoom in or out. You simply zoom out to see your position relative to the coast etc then zoom back in to see the detail of pilotage info, depths and all the other stuff. The battery is waterproof and lasts many many hours but I would turn it off whenever I was not looking at it to conserve power on the dinghy. You can buy extra battery packs but they are very expensive. On the yacht it sits in a mount when using it in the cockpit and this mount charges it from the yacht battery. You could do the same on a Wayfarer. I would take a small fully charged 12v gel battery (or sealed lead acid) and connect it to the mount. Yuasa make many such batteries and they are inexpensive and would run the plotter for days and days. I once bought one for something else that was about 6 x 4 x 2 inches and it cost about £25. One word of warning, it may be waterproof but I do not think it floats! It cost over £700 with all the charts for UK and Ireland but they are often sold off as new models come out every couple of years. You can still find mine new for half the original cost and second hand units are even cheaper.
Hope this helps.