Surprise Lake Hot Spring
Surprise Lake Hot Spring is inside Aniakchak National Monument, on Park Service ground, Dillingham Census Area, with Dillingham itself about 150 miles distant. the access on record says open. The water was sampled in July 1976 and carries iron in the mix, but no temperature was taken, so the record gives no answer on soaking. Nobody logged a pH either; the sample covered dissolved solids and minerals.
Sixteen miles to the nearest road. National monument backcountry with no named trail in the record. Elevation is low, and the climate is wet and snowy, with over 160 inches of snow in a season and an average temperature a little above freezing. No campground is attached, and no fee.
Summer, with no best-month data to narrow it further. Expect to be on your own entirely; the file lists no cabin and no fee. Monument rules apply, so check with the National Park Service about permits. The 1976 chemistry is the only water data, and it says nothing about heat.
The water at Surprise Lake Hot Spring contains 548 mg/L total dissolved solids. It is notably rich in magnesium (known for muscle relaxation and stress relief) and iron (iron-rich springs may support circulation).