sushi, seasoned rice
鮨 centers on sushi, specifically the combination of fish and seasoned rice. The meaning extends naturally to the seasoned rice itself as a key component.
鮨 combines 魚 (fish) with 旨 (delicious, tasty), reflecting the idea of fish prepared in a delicious way, historically referring to fermented fish with rice. The exact historical development of the character is uncertain.
The left side is a fish 魚, and the right side is 旨 (delicious). Picture a delicious piece of fish on seasoned rice: that's sushi.
For シ, imagine a sushi chef saying 'she' makes the best sushi: 'she' -> シ, tying the sound to the delicious fish and rice.
narezushi; fermented sushi (pickled in brine rather than vinegar), precursor of modern sushi
Kansai-style sushi (esp. pressed sushi, such as battera)
makizushi comprising mackerel fillet marinated in salt and vinegar, wrapped in rice and covered in shredded kombu
narezushi of amberjack and turnip (Kanazawa speciality)
kimizushi; sushi with egg yolk instead of rice