Okay, as I understand it you have a category 6A cabin. Since you didn't know your cabin number before, I assume you booked a guarantee cabin. I don't know if you booked and inside guarantee, in which case you were upgraded, or an ocean view guarantee, in which case you got what you booked, but at a slight discount.
I have not been in a 6A on any Carnival ship, though I have been in a 6B and the equivalent of a 6A on a similar sized ship owned by Royal Caribbean (it was the Monarch of the Seas). From my personal experience while you are in the cabin you can't tell the difference between a cat 6A and and cat 6D.
There are two advantages to being on the lowest deck. When the ship is docked (except for the home port) or when anchored (meaning you tender to shore on one of the lifeboats) you are closer to the deck where you board and/or disembark the ship.
The other advantage is because you are closer to the water line, there is less roll (side to side motion) of the ship.
Also, you are not on the lowest deck. You are on the lowest passenger deck, but the crew sleeps below you, so there are decks below you.
The disadvantage of being on the Riviera decks is that everything on the ship is above you - several decks above you. If you like climbing stairs, that can be a good thing.
And finally, being down low means that during high tide, if the water gets high enough, then you get to look out your window and see the fish swimming along the ship - kind of cool actually.
