Storing natural gas in the form of clathrate hydrates (termed as Solidified Natural Gas, SNG) is highly advantageous as it is non-explosive, environmentally benign and offers compact mode of natural gas storage with high volumetric storage capacity. In this work, we demonstrate rapid methane storage in saline water (1.1 mol% NaCl solution) and seawater via clathrate hydrates aided by 5.56 mol% THF in a simple unstirred tank reactor. We report extremely fast hydrate formation kinetics with methane uptake of 89.2 (+/- 2.4) v/v in 13.8 (+/- 2.4) minutes with saline water and 86.3 (+/- 4.3) v/v in 15.1 (+/- 0.8) minutes with natural seawater (t(90)). This uptake corresponds to an yield of 77.6 (+/- 2.2)% for saline water and 75.0 (+/- 3.4)% for natural seawater system respectively for the stated hydrate growth time. Further, molecular insights of the mixed hydrate formation in presence of NaCl is derived through high-pressure calorimetry, in-situ Raman, and powder X-ray diffraction analysis. Finally, we demonstrate the stability of the hydrate pellet formed employing direct seawater in presence of THF for two weeks. The direct use of natural seawater makes the SNG technology highly attractive to store/transport methane for large-scale storage needs and for low capacity natural gas production facilities like biogas manufacturing plants.