Hi, nilla and welcome to a fellow Brit!

All I can find on Potomac confirms that it is Native American in origin:

Potomac: River forming the boundary line between Maryland, Virginia, and West Virginia. Indian Patowmek or Potowmak, "they are coming by water"; another translation, "place of the burning pine," allusion to a council fire.

Can't explain why there are two wildly different translations. Maybe the name occurs in two tribal languages with different meanings. There will be others who can give us more information I'm sure.