Paralilly

The double flower are a floral species that float on the sea surface, preferably near cliffs or beaches. Each surfacing paralilly flower has an underwater twin. If one is kept underwater and the other in the air, they exchange fresh air with fresh waters. If the twin dies, so does the connection. A paralilly flower will suck itself onto any surface it comes into contact with, creating a seal between mouth/nose and water or gills and air. 
Paralillies are a popular method for humanoids to breathe underwater and for aquanoids to breathe in air. In touristic areas where many land walkers visit a body of water or where swimmers visit the land, business can be found that guard paralilly twins for financial compensation.