The chapter introduces the classes of hydro-aviation aircraft, discusses the estimation of their gross weight, their maximum lift capacity, maximum range, and the probability of their operation in the required season and site in the ocean. The chapter offers a series of design recommendations for seaplane design to overcome wind waves, to operate safely in crosswinds, intense fog, and poor visibility. The chapter introduces basic structural design concepts and some recommendations for a jet multirole bottom-amphibious aircraft of Beriev Be-200 type. Finally, the chapter presents a concept of seaworthiness and a practical algorithm for determining the geometry of the bottom of an amphibious aircraft.