The canals in Venice reflect the sky, and imitate whatever color the sky is. No surprise there. On overcast days, you see waves of white that calm and vanish on the horizon to meet the white sky. If it was any brighter, it would hurt to look at, but as it is, it is beautiful in its blankness and motion. It is like watching undulating snowbanks.
But when it is sunny and clear, the water has a pale azure cast. It doesn't quite glow and isn't as brilliant as the water in the Blue Grotto, but it is still like a jewel to behold. The advantage is the canals are all around you. You don't have to take a bus, pay a fee, then take a rowboat to see the color for a few minutes.
It's a question of whether you want to have to journey and make a special effort to have a peak experience for a few minutes, or whether you want the less intense beauty integrated into your daily life.