Here is a new location that should be added to the map, I discovered it today and posted it on the Oblivion wiki today at:
It is very useful.
There is another entrance for the Wizard's Grotto, this one doesn't require the castle. To take this route you are going to want an underwater breathing spell, one can be obtained for free in one of the recommendation quests for the mage's guild. First go outside of bravil and swim directly to the right of the word "Bravil", the entrance is located on the "o" in "Nibien Boy". It is a trap door hidden in between some rocks, it leads you right into the underwater part of the Wizards Grotto. [Alexander Michael Patterson] - Team Xlink