[3.5] Ways to learn spells from a different class list

[Curmudgeon]

Except for the fact that control water is either a 4th or 6th level spell.

Control Water is a 4th level Druid Spell and a 6th level Sorcerer spell, but that says nothing about its Bard level.

Also, a bard is incapable of casting 42nd level spells.

What does that have to do with Extra Spell? Should a Ranger get Miracle as a level 4 spell because they're incapable of casting 9th level spells? "42" is the Answer to the Ultimate Question of Life, the Universe, and Everything. That, at least, is a citable reference. What rule can you cite which gives the corresponding Bard level for a Druid spell?