Water has such a huge role in home canning, yet we take it for granted.
There are three issues to consider:
1) Clean, safe, potable water (See page on bacterial load.)
2) Hard water (See page on hard water.)
3) Using fresh water instead of blanching water to fill the canning jars, to reduce starch density inside the jar on some blanched items such as potatoes. (See page on fresh water in jars.)
See also the page on: Acidity of water and vinegar combinations.
Note as well that canning directions for fish and seafood explicitly advise against using stream, lake or seawater either for washing the fish or seafood in, or, for filling the jars with. They strongly recommend that you use potable tap water instead.