Whether you ship by air or sea, so long as you do it consistently, the number of phones you have available is a constant based on the rate you're manufacturing them. All the air versus sea argument does is change the lag in receiving them.
Assuming the rate they can be manufactured can be adjusted based on demand, you want to make sure you're always on the verge of selling out to avoid carrying more inventory.
So my guess is that they normally ship by sea but increase the manufacturing rate and ship some percentage of phones by air to keep the stores "just stocked enough".
http://www.bloomberg.com/news/articles/2015-02-02/apple-said...
For 30 days, it would of course be even lower, but not zero.
However, the real issue is the depreciation on components makes shipping cutting edge electronics by sea untenable. Put another way Apple does not want to wait another 25 days before releasing their new phone or be forced to ramp up production on an even less proven technology.
PS: Another option is to ship the phones by air but the packaging by sea. However, phones are small enough that it would not be woth maintaining a U.S. factory for final assembly.
Taxing money coming into a country is really bad economics and amounts to just pure greed.
Countries should want money coming into their markets. Not punishing it.
I would also be more convinced of the idea of not taxing the money coming in to the USA if it had actually been taxed in the countries where the profits had been made - let me introduce you to the Double Irish Arrangement [1].