Ten thousand years or so after the first humans built sailing ships for trade, the coast still matters immensely. In fact, there are only two prosperous landlocked countries of any size: Austria and Switzerland.
Geography does not drive economics. Look at Japan, Iceland, Singapore, Hong Kong, Finland. None are geographically rich, but the people are. Winston Churchill and his friends raved about the wealth of Africa, as Uganda and Kenya have fertile lands, lots of minerals ... and they are economic wastelands now.
But back to Megan's point. Non-communist Europe has 15 countries, and the two landlocked countries (Aus and Switz). The other 13 countries are prosperous, and have coasts, but it is not their coastliness that matters. Adopting free market policies, avoiding blatantly corrupt governments, is key (unlike the subtly corrupt Western governments). The problem with Zimbabwe or Uzbekistan is not their lack of regattas.
In North America, Canada and the US are on coasts, but then so is all of central America, which is dirt poor. South America has 13 countries, and the 2 landlocked ones are poor, but so are most of their countries on the coast, and with only 2, the standard error on this pattern is large. Most landlocked countries are either in Africa, or ex-communist states, and that's the real pattern, not water. But then that's not very interesting, because the problems of Africa, and the ex-Soviet satellites are pretty different.
In warfare, it is useful to remember that the tough terrain, geographically, is easier to go through than a heavily defended terrain. That's why you go through the Ardennes forest and not the Maginot line. It's people and the things they create that matter, not geography.