The single, enormous leaf on a Lotad's head is nearly encircled by a raised, solid ridge, with a small triangular indentation in the back. The leaf is tilted slightly towards this indentation, for rainwater must drip from this leaf onto a Lotad's permeable back to provide them with the nutrients they need to survive. Pouring water directly onto their bodies is useless; the water must be pure and drain from their leaf in order to nourish a Lotad.
Older, larger Lotad gain extra energy by acting as ferries for small and starving pokemon, who give them what little food they have left to be allowed to ride their leaf across a river or small stretch of sea. It is easy for humans to mistake the sight of a group of Lotad floating across a river, carrying tiny pokemon such as Wurmple or Skitty on their leaves, for a peaceful sign of nature's beauty. For the pokemon riding them, however, it is a grim and dangerous voyage of exploration.
A Lotad's leaf is slippery and tilted towards the back; staying on board is a constant and strenuous effort. Lotad make no efforts to pick up drowned passengers, for allowing their leaves to touch river water would expose them to dangerous microbes, rendering them incapable of processing rainwater. Few survive this journey, but their progeny soon multiply into flourishing new colonies, for Lotad are such knowledgeable navigators that some pokemon continue to use them even when boats are available.
Perhaps it is this dangerous journey taken by so many pokemon into the unknown which inspired the ancient belief in Hoenn that in order to be reincarnated, one must first ride a giant Lotad across the river of the dead.