This paper presents a comprehensive survey on semantic communications in the Space-Air-Ground-Sea Integrated Network (SAGSIN), focusing on the new paradigm, frameworks, and challenges. The SAGSIN is envisioned to address communication coverage challenges in difficult environments such as forests, deserts, and seas, by integrating space, air, ground, and sea networks. However, traditional communication technologies based on Shannon information theory are insufficient due to the large-scale scenarios, highly dynamic channels, and limited device capabilities in SAGSIN. Therefore, semantic communications, which focus on exchanging semantics rather than bit-level reconstruction, are proposed as a solution. Semantic communications are defined from three perspectives: significance, meaning, and effectiveness. Based on these, three types of semantic communication systems are introduced: significance-oriented, meaning-oriented, and effectiveness/task-oriented. These systems require a new integrated paradigm called Perception-Communication-Computing-Actuation Integrated Paradigm (PCCAIP), which combines perception, computing, and actuation techniques to facilitate significance-oriented sampling and transmission, semantic extraction and reconstruction, and task decision. The paper also discusses the challenges of semantic communications in SAGSIN, including the need for robust communication in highly dynamic channels, efficient resource utilization, and addressing security issues. It highlights the importance of semantic communications in future 6G networks, where traditional bit-level communication may not be sufficient. The survey provides a comprehensive review of semantic communications in SAGSIN, including performance metrics and techniques for further research.This paper presents a comprehensive survey on semantic communications in the Space-Air-Ground-Sea Integrated Network (SAGSIN), focusing on the new paradigm, frameworks, and challenges. The SAGSIN is envisioned to address communication coverage challenges in difficult environments such as forests, deserts, and seas, by integrating space, air, ground, and sea networks. However, traditional communication technologies based on Shannon information theory are insufficient due to the large-scale scenarios, highly dynamic channels, and limited device capabilities in SAGSIN. Therefore, semantic communications, which focus on exchanging semantics rather than bit-level reconstruction, are proposed as a solution. Semantic communications are defined from three perspectives: significance, meaning, and effectiveness. Based on these, three types of semantic communication systems are introduced: significance-oriented, meaning-oriented, and effectiveness/task-oriented. These systems require a new integrated paradigm called Perception-Communication-Computing-Actuation Integrated Paradigm (PCCAIP), which combines perception, computing, and actuation techniques to facilitate significance-oriented sampling and transmission, semantic extraction and reconstruction, and task decision. The paper also discusses the challenges of semantic communications in SAGSIN, including the need for robust communication in highly dynamic channels, efficient resource utilization, and addressing security issues. It highlights the importance of semantic communications in future 6G networks, where traditional bit-level communication may not be sufficient. The survey provides a comprehensive review of semantic communications in SAGSIN, including performance metrics and techniques for further research.