Carrying O2 in water doesn't give any real advantage as it has a such limited usage window (you have a deco station in the water, doesn't make sense to carry one). You will carry whatever bottle you'll need for your dive, however if I wold be forced to chose a "generic" one, I will go for EAN50-EAN66.
O2 on the boat is another matter.
O2 on the boat is another matter.