One reason is that saltwater would be impossible for them. Prolonged exposure would drain what little moisture they had in them, and they would dry up and be unable to do anything. It would, effectively, kill the brain.

Secondly, in any water, they would float, not sink.
Despite the lack of air in the lungs, after death decomposition sets in and all sorts of gases are released. It is true that after a while, the body sinks, but obviously zombies decay slower than a normal cadaver.

Thirdly, digestion also produces gases. And since a human stomach really isn’t all that big, zombie must be able to digest. Otherwise they’d be unable to eat, and obviously they can do that.

Fourthly, the pressure, particularly in deep oceans, would crush the zombies. Literally, crush.

Fifthly, as you might now, a corpse left in water for too long becomes extremely bloated. A zombie like that would barely be able to walk, and would only be dangerous if you came right up to it.

Sixthly, any moving body of water will have a current. In rivers, this is accompanied by assorted rocks and pebbles, which would shred a zombie. In oceans, this current would be strong enough to drag the zombie without giving it a fighting chance.

As such, a defense complex underwater in an ocean would be the perfect defence place.

I never considered an underwater base, because in my opinion the disadvantages outweigh the advantages. As well as those mentioned in the article, you would also find it much more difficult to grow most plants, and would probably end up malnourished. Also, If it is underwater, it would be difficult to escape if the complex became over-run.

However, reading this led me to consider the possibility of using large ships, like aircraft carriers. You could keep animals, and create a patch of soil on the deck to grow plants. Obviously, you’d be able to use the animal waste as manure, making the plant patch sustainable. Also, with the proper facilities, you have all the water you can drink. It wouldn’t be difficult to purify the sea water.

Provided you avoid major storms, it would be impossible for zombies to get on board. The decks are high above surface level, and any ladders on the side could be removed. If you were lucky enough to find an aircraft carrier which still had aircraft on, and happened to have someone who can fly, you could use the planes/helicopters to scout out islands and search coastal cities for besieged survivors which you might be able to rescue.
Fuel would be an issue, but it wouldn’t be hard to make your own mast and sails, provided you know what you’re doing. I’m sure there’s a guide on the internet somewhere. This way you can conserve fuel for use solely on refridgeration (useful, eh?) and medical services. The large amount of fuel on board would probably last for several years if used sparingly, and it wouldn’t be too hard to scavenge fuel from abandoned vessels in a harbour.

An additional note i would like to add- if you were to head towards either the north or south, it would eventually be cold enough that you wouldn’t need refridgeration to store perishable food, which could prove useful. Also, i doubt there would be many zombies at the south pole. However, the disadvantages offered by the cold temperature may outwiegh the advantadges.