Before you decide on getting an octopus, have you considered:
-That you can't have anything else?
-That you can afford to give ir live foods?
-That they don't live very long?
-That they are hard to keep?
-That you need a sump?
-That you won't see it every day?
That being said, maybe an octopus mercatoris will be fine, but those are nocturnal. The only other kinds would be octopus chierchae and blue ring octos, but octopus chierchae would be better donated to a lab and blue rings are EXTREMELY poisonous, and so are never recommended. However, I've heard octopuses are very rewarding, so I wish you luck.
On a side note, how long have you kept saltwater tanks/reefs?