Seawater has 'critters' in it. If you collect it and leave it in a barrel the critters will die and fall to the bottom, no big deal. They will also degrade, adding a small amount of nutrients to the water. Unless you seal the water container, or there is a huge amount of 'critters' in the water the water is not likely to go 'anaerobic'. What I mean by this is that if seawater with any significant amount of 'stuff' in it is kept in a low or very low dissolved oxygen condition, then the anaerobic bacteria in the water will start to use sulfate as an electron receptor (or is it donar, I don't remember)...anyhow, when they do this the sulfate will be converted to hydrogen sulfide, of which our human noses are very sensitive to (rotten eggs smell). So............if I were to collect NSW (much too much trouble for me) I would just put it in a big loosely covered barrel. After everything died I would just siphon out the water trying not to disturb the debris on the bottom. If I smelled hydrogen sulfide I would toss in an airstone for a few days until I didn't smell it any longer. You could also leave an airstone in continuously after collection if you though there were a lot of 'critters' in the water.