shallow, not deep, superficial
淺 centers on lack of depth: physically shallow water, or metaphorically superficial and not deep.
淺 combines 氵 (water) with 戔, which likely contributes the sound and the idea of smallness or shallowness. The exact historical development of 戔 is uncertain.
Water 氵 beside 戔 suggests a small amount of water: a shallow stream or puddle. The right side looks like stacked small things, reinforcing the idea of not deep.
For セン, imagine a shallow stream where you can see a cent (coin) at the bottom: cent -> セン, and the water is so shallow the coin is visible.