In New Pokemon Snap, travel to the islands that make up the Lental region. There are dozens of Pokémon roaming around Lental island. The request is to take a pose of Grateful Lanturn.
A Light in the Depths in New Pokemon Snap
Guide
Visit Lental Seafloor (undersea) at research level 3.
Take the alternate route that leads to the underground ruins.
Throw an Illumina Orb at Lanturn while descending down to the Undersea Cave floor
Once the Wailord passes, you’ll see 2 Frillish holding the Lanturn.
Hit the Lanturn with an Orb to make him break free from Frillish’s grasp.
Once you reach the seafloor, you’ll see the Lanturn turnaround and face you.
Snap a Photo of Grateful Lanturn to fulfill the request.
NOTE: After expressing gratitude, the Lanturn will swim inside a small cave to your right. You’ll now be able to unlock the alternate route (into Lugia’s Lair).