•Catalytic “peak HPS” was produced in the solution of κ, ι-, and λ-carrageenan.•Peak increased at negative accumulation potentials due to breakdown of associations.•AdT CPS conditions for κ, ι-, and λ-carrageenan analysis in seawater were defined.•Only λ-carrageenan could be detected in the mixture with κ- and/or ι-carrageenan. Sulphated polysaccharides κ-, ι-, and λ-carrageenan were analysed in seawater by applying adsorptive transfer constant current derivative chronopotentiometric stripping (AdT CPS) method on mercury electrode. Accelerated faradaic (catalytic) nature of the peak (peak HPS) recorded at negative potential was confirmed for all three carrageenans using specific electrochemical criteria. The effect of chronopotentiometric parameters (accumulation potential, accumulation time and stripping current) on the peak HPS height was evaluated for each carrageenan. Optimal CPS parameters for AdT CPS analysis of particular carrageenan in seawater were defined. Regarding the AdT CPS of carrageenans mixture, only λ-carrageenan could be successfully measured in the mixture with ι- and/or κ-carrageenan. This is the first electroanalytical study of a model mixture of sulphated polysaccharides in seawater and it is of great importance for the analysis of organic matter in seawater, especially acidic polysaccharides containing sulphate groups in the structure.