The Scarlet, a new 36-room eco-hotel on England North Cornwall coast, is notable not so much for its rich modernist details, infinity pools, giant glass walls with views of the beach below but for its thoughtful eco-minded design. Set on National Trust-protected land, the hotel has an indoor pool heated using solar panel energy and an outdoor swimming area filled with captured rainwater and filtered by reeds and other sieving plants that stand in for chlorine. Log fires warm the giant red teacup-shaped seaweed hot tubs overlooking the sea. The hotel also harvests its gray water, which is used for...