space-air-ground-sea integrated (sagsi) networks are envisioned to connect satellite, aerial, ground, and sea networks to provide connectivity everywhere and all the time in sixth-generation (6g) networks. however, the success of sagsi networks is constrained by several challenges including resource optimization when the users have diverse requirements and applications. we present a comprehensive review of sagsi networks from a resource optimization perspective. we discuss use case scenarios and possible applications of sagsi networks. the resource optimization discussion considers the challenges associated with sagsi networks. in our review, we categorized resource optimization techniques based on throughput and capacity maximization, delay minimization, energy consumption, task offloading, task scheduling, resource allocation or utilization, network operation cost, outage probability, and the average age of information, joint optimization (data rate difference, storage or caching, cpu cycle frequency), the overall performance of network and performance degradation, software-defined networking, and intelligent surveillance and relay communication. we then formulate a mathematical framework for maximizing energy efficiency, resource utilization, and user association. we optimize user association while satisfying the constraints of transmit power, data rate, and user association with priority. the binary decision variable is used to associate users with system resources. since the decision variable is binary and constraints are linear, the formulated problem is a binary linear programming problem. based on our formulated framework, we simulate and analyze the performance of three different algorithms (branch and bound algorithm, interior point method, and barrier simplex algorithm) and compare the results. simulation results show that the branch and bound algorithm shows the best results, so this is our benchmark algorithm. the complexity of branch and bound increases exponentially as the number of users and stations increases in the sagsi network. we got comparable results for the interior point method and barrier simplex algorithm to the benchmark algorithm with low complexity. finally, we discuss future research directions and challenges of resource optimization in sagsi networks.