fish basket kept in water
罩 centers on a basket-like container used to hold fish while keeping them in water, functioning as a submerged trap or live well.
罩 combines 罒 (net) with 卓, which likely contributes the sound and may suggest a raised or prominent basket. The character specifically refers to a fish basket kept in water.
The top 罒 is a net, and the bottom 卓 looks like a table or stand. Imagine a net-covered basket standing in the water, holding fish like a table holds objects.
For トウ, picture a fish basket floating in a towed boat: tow -> トウ, and the basket keeps the catch fresh in the water.