There are two Hot Springs in the Iki Island Expansion for Ghost of Tsushima: Director’s Cut. These are completely optional but if you’re aiming for all Trophies, must be completed towards that goal. These will also offer a minor Health increase to Jin.
Gonoura Cape Hot Spring - South of The Blood-Stained Shrine
The first Hotspring can be found at Gonoura Cape, to the far south of it. If you’ve already unlocked the Blood-Staned Shrine, it’s just a short trip to the south of it.
Senjo Gorge Hot Spring - West of Kazumasa’s End
The second Hotspring is a short ride to the west of the Kazumasa’s End point of interest. Simply follow the path to the west to locate it, you’ll have to scale the cliff to reach the Hot Spring on top.