In order to collect sea water, you need a small crystal flask. Stand (you cannot be sitting or kneeling) in the room where you are doing the collection, hold your flask, and HARVEST WATER WITH FLASK. This will fill your flask with 20 doses of sea water.
In the past, sorcerers had to use a vial to collect the sea water and pour the vial into the flask repeatedly until the flask was full. Flasks were allowed to collect sea water with the introduction of Alchemy.