sushi, seasoned rice
鮓 centers on sushi and seasoned rice, specifically the older style of fermented fish and rice that evolved into modern sushi.
鮓 combines 魚 (fish) with 乍, which likely contributes the sound. The character originally referred to a fermented fish and rice dish, an early form of sushi.
The left side is a fish 魚, and the right side looks like a container or preparation surface. Picture fish being pressed with seasoned rice to make sushi.
For サ, imagine a sushi chef saying 'sa!' as they press the fish onto the rice: sa -> サ.