You don't want heavy cylinders in the ocean. I do dive steel cylinders, but I keep it to LP85s which are light enough I can swim them to the surface if I needed to. They also have plenty of volume, I can comfortably do a 25 minute dive at 200' with them.
Deco cylinders I'm using AL40s in the ocean. Works pretty well too.
Deco cylinders I'm using AL40s in the ocean. Works pretty well too.