Sorry - missed that in my search of bugs. Never saw it before 1.8. too.
But the Wiki also states:
--> Elder Guardians' eyes will still follow a completely unarmored player under the effects of a Potion of Invisibility, but this is entirely visual and they won't attack them.
So I assume that being invisible would allow the character to approach the underwater temple.
Mining fatigue "damage" does not require "line-of-sight" to occur. This is a bug. Invisibility is just that - invisible. wiki makes that clear. This should be marked unresolved.