ditch, sluice, gutter, drain, blaspheme, pollute
瀆 centers on a channel for water: a ditch, sluice, gutter, or drain. The extended meanings of blasphemy and pollution likely come from the idea of defiling or making something impure, as if pouring filth into a clean channel.
瀆 combines 氵 (water) with 賣, which likely contributes the sound. The water radical points to a water channel, while the extended meanings of defilement may be a semantic extension from dirty water.
Water 氵 flows through the right side 賣 like a channel. Imagine a ditch or gutter carrying water, but if you pour something foul into it, you pollute the channel and blaspheme its purity.
For トク, imagine a talking drain that says 'talk' when water flows: talk -> トク, and the drain is a 瀆.