While Samantha isn't Welsh, there are some Welsh names that have similar meanings:
* Gwenllian: "White wave"
* Eirwen: "White snow"
* Ceri: "Beloved"
* Llinos: "Flax"
* Gwen: "White"
If you're looking for a Welsh name with a similar sound, you could consider Sian or Siân, which is a Welsh form of Jane or Joan.