Remember that ships typically sail under the flag of the country they are registered to, so unless Google is forming their own country or becoming pirates, they are not jurisdiction-free. There is also question of bandwidth.

On the whole, property tax is a very small expense for a data center. Most of them are leased anyway, so the data center doesn't actually pay property tax in the first place (it's built into the cost just like the building itself.)

The patent application also doesn't say which country they might want to park these things off the coast of. With longer range wireless options coming on the market, why not park one just outside Shanghai and Taiwan?

RE: Google planning offshore water-based data centers