The speed math
A boat on regular ice is fast; on packed ice faster; on blue ice it reaches the maximum boat speed — roughly 70+ blocks per second in a straight tunnel. A blue ice highway through the Nether (1:8 ratio) moves you across thousands of Overworld blocks in under a minute.
The tunnel build
A 2-high tunnel, blue ice lane in the floor (9 packed ice around 1 blue ice... practically: blue ice crafted from 9 packed ice), walls to keep the boat in line, and side ice at intervals for braking. Light it dimly (ice does not need light) and add corner walls — flying off a turn at max speed is the classic crash. Glass roof keeps it mob-proof and lets you watch the view.
Material cost strategy
Blue ice is expensive per block (9 packed ice each, and packed ice needs 9 regular ice). Strip the lane as pure blue ice only on main highways; use packed ice for branch lines. Ice collection needs the Silk Touch enchantment or colder-biome mechanics — silk touch is the practical route, so bring the pickaxe you enchanted first.
