Another use for foils is in the water. Relying on the exact same principles as air foils, just with liquid water as the fluid instead of air, the development of hydrofoils has allowed for the lifting of the hulls of boats out of the water.  One main difficulty with hydrofoils is cavitation. This is basically the creation of bubbles in the liquid not due to heat but due to low pressure. This rapid reaction can cause temperatures up to 10,000 K and heat nearby surfaces 500 K. Additionally this can create high pressures and shockwaves through the fluid. These shockwaves can cause the collapse of other bubbles in a type of chain reaction. These extreme circumstances can cause erosion of the foil itself. Also the cavitation increases drag on the foil and thus destroys the lift of the foil. Careful engineering can keep this from happening and, by balancing the lift of the foils with the gravitational force of the boat the boat call sit completely onto only the foils themselves which reduces drag and allows for much greater velocities.
     Some of these foiling boats use wings for sails as well which utilizes an air foil to generate thrust for the boat in a much more effective way than a normal sail. This is like the difference between using an airfoil kite and a standard diamond frame kite. Using both technologies these sail boats are potentially the most efficient method of transportation.

https://www.npr.org/sections/thetwo-way/2017/11/21/565604657/americas-cup-race-gets-a-radical-new-single-hulled-boat

http://sailing-news.com/fr/1/permanent/32261/gc32-catamaran-austria-cup-traunsee-aut-day-1.html

http://sailing-news.com/fr/1/permanent/32261/gc32-catamaran-austria-cup-traunsee-aut-day-1.html

https://whatisthebeststuff.com/outdoors/hydrofoil-boarding

https://www.vendeeglobe.org/en/news/16219/the-brazilian-job

https://www.dailymail.co.uk/sciencetech/article-4228566/Airbus-Oracle-unveil-flying-yacht-America-s-Cup.html

This is a quick video of the result that decades of research in the physics of lift in both water and air has cumulated to. The ability for a sailboat powered by nothing but the wind to sustain speeds close to 60mph is an incredible achievement and I think that this has great potential for very sustainable and efficient travel. They have made planes which can soar so well that they can make it around the world with minimal energy consumption in comparison to a standard airliner. I think that foil technology is a big part of the future be it in the air or the water.