Tethys (Τηθύς) is the Greek Titaness of fresh water and the sea — sister-wife of Oceanus, mother of all rivers and the three thousand Oceanids.
Saturn's moon Tethys is named for her.
Tethys does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Tethys reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.