He is in Shiver Snowfields and you will locate him at night on one of the alternate routes, but you must unlock the alternate routes in Shiver Snowfields, in order to get there.
Go to the alternate routes and so you will be close to the ice floes, and throw an Illumina Orb at the Jynx that is above the Avalugg.
Keep in mind that Jynx will move her arms, and you will see Suicune, so you have to act fast, because Suicune goes fast in the water, and you are in the last area of the route.
We hope that with this guide on where to find Suicune in New Pokemon Snap, you can locate this legendary dog in a way that you will have fun in the game.