go upstream
泝 centers on moving against a current: going upstream or tracing a river back toward its source.
泝 combines 氵 (water) with 斥, which likely contributes the sound and the idea of pushing back or opposing, together suggesting moving against the water flow.
Water 氵 on the left and 斥 (repel, reject) on the right: imagine water pushing back as you try to go upstream, repelling your advance.
For ソ, picture a salmon swimming upstream: the first syllable 'so' in 'salmon' cues ソ, and the fish fights the current just like 泝.