eventide, tide, salt water, opportunity
汐 centers on the rhythmic movement of the sea: the tide, especially the evening tide. The 'opportunity' sense is a natural extension, like a favorable tide that brings a chance.
汐 combines 氵 (water) with 夕 (evening), suggesting the evening tide. The character likely originated as a way to specify the tide that comes in the evening.
Water 氵 and evening 夕 together: picture the evening tide rolling in as the sun sets. The tide brings both salt water and opportunities.
For セキ, imagine a seasick person watching the evening tide: 'seasick' cues セキ, and the tide makes them queasy.