Peel, pit, and dice the mangos. Transfer mango chunks to a blender or food processor and blend into a very smooth puree.
For extra smoothness, run the mango puree through a fine sieve, discarding any fibers.
In a large bowl, combine cold water and gelatin. Mix for 30-45 seconds until thickened.
Add sugar, coconut milk, and mango puree to the gelatin mixture. Whisk until smooth.
Transfer to a small pot and heat over medium-low until the gelatin is fully dissolved, ensuring the mixture does not boil.
Pour into serving bowls or a mold, removing air bubbles if necessary. Optionally, cover with plastic wrap.
Refrigerate for at least 6 hours, or overnight, until set.