This paper explores the integration of space-air-ground-sea (SAGSIN) networks to address communication coverage challenges in difficult environments such as forests, deserts, and seas. Traditional Shannon-based communications are insufficient due to large-scale scenarios, highly dynamic channels, and limited device capabilities. The paper introduces semantic communications, which focus on high-level information exchange rather than bit-level reconstruction, to enhance communication efficiency and effectiveness. It reviews three perspectives of semantic communications: significance representation and protection, meaning similarity measurement and enhancement, and effectiveness estimation and yield. Three types of semantic communication systems are introduced: significance-oriented, meaning-oriented, and effectiveness/task-oriented. The implementation of these systems requires a new paradigm called Perception-Communication-Computing-Actuation Integrated Paradigm (PCCAIP), which integrates perception, computing, and actuation techniques. The paper also discusses future challenges and provides a comprehensive review of semantic communications in SAGSIN, emphasizing the importance of performance metrics and techniques.This paper explores the integration of space-air-ground-sea (SAGSIN) networks to address communication coverage challenges in difficult environments such as forests, deserts, and seas. Traditional Shannon-based communications are insufficient due to large-scale scenarios, highly dynamic channels, and limited device capabilities. The paper introduces semantic communications, which focus on high-level information exchange rather than bit-level reconstruction, to enhance communication efficiency and effectiveness. It reviews three perspectives of semantic communications: significance representation and protection, meaning similarity measurement and enhancement, and effectiveness estimation and yield. Three types of semantic communication systems are introduced: significance-oriented, meaning-oriented, and effectiveness/task-oriented. The implementation of these systems requires a new paradigm called Perception-Communication-Computing-Actuation Integrated Paradigm (PCCAIP), which integrates perception, computing, and actuation techniques. The paper also discusses future challenges and provides a comprehensive review of semantic communications in SAGSIN, emphasizing the importance of performance metrics and techniques.