The book presents the most relevant research of the participants of the VII International Conference of Young Scientists "Complex Investigation of the World Ocean" (CIWO-2023). This conference was held at Saint Petersburg State University in May 15-19, 2023 (Saint Petersburg, Russia). It covers a wide range of fundamental and applied marine and limnology studies combined in eight sections: Ocean Physics, Ocean Biology, Ocean Chemistry, Marine Geology, Marine Geophysics, Marine Ecology and Environmental Management, Physical and Biological interactions (interdisciplinary section), Oceanological Technology and Instrumentation. The aim of this book is to show the relevance of the marine research due to the crucial role of the World Ocean in determining climate change on Earth, huge resources (fish resources, oil, gas and ore deposits, etc.) and intensive development of infrastructure in coastal and offshore zones. All these topics were marked within the framework of realization of the United Nations Decade of Ocean Science for Sustainable Development (2021-2030).  The studies presented in the book covers the wide spectrum of different the most important marine and limnology issues: thermohaline structure of water body and interactions between ocean and atmosphere, dynamic of the ocean, marine ice in polar regions, biodiversity of the marine ecosystems, adaptation of marine life to climate changes, geological and geophysical investigations in oil and gas regions, sedimentation, paleooceanology and biostratigraphy, hydrochemistry of estuary regions and carbon fluxes, microplastic pollution of the ocean, eutrophication and etc.