This book is a preface to modern marine structures. Marine structures like as underwater tunnels are new patents that are developing around the world. In this book, first chapter describes general properties and different types of underwater tunnels. Second chapter specially describes submerged floating tunnels and third chapter is about principals of hydrodynamic properties of submerged floating tunnels. In appendix present, some designs for modern marine accommodations like as underwater restaurants and hotels. Some of them are just concepts and others are prototypes. This book will be used by marine designers, architectures and structure engineers.