waves, billows
涛 centers on large, rolling water movements: waves and billows. The idea is of water in powerful, surging motion.
涛 combines 氵 (water) with 寿, which likely contributes the sound and may suggest long-lasting or sustained motion, giving the idea of enduring waves.
The water radical 氵 beside 寿 (long life) suggests waves that seem to live forever, endlessly rolling and surging.
For トウ, imagine a towering wave: tower -> トウ, and the wave rises high like a tower.