big rain, swamp
沛 centers on abundant water: a heavy downpour or a waterlogged swamp. Both meanings share the idea of overwhelming wetness.
沛 combines 氵 (water) with 巿, which likely contributes the sound. The character expresses abundant water, fitting both heavy rain and swampy ground.
Water 氵 pours down on the right side 巿 like a cloth being drenched. Imagine a sudden big rain turning everything into a swamp.
For ハイ, picture a high tide flooding the swamp: high -> ハイ, and the water rises everywhere.