Why are landlocked countries generally poorer than countries with access to waterways?

With a few exceptions the world's 45 landlocked countries are poor. In spite of technological improvements in transport, landlocked developing countries continue to face structural challenges to accessing world markets. As a result, landlocked countries often lag behind their maritime neighbors in overall development and external trade.

Foe countries, easy and free access to waterways (navigable rivers and oceans) has always made trade more profitable because sea transportation can reach other markets faster and farther away. In trade, every extra day added to make goods reach their destination costs more money which is added to the retail price turning their purchase less attractive. Besides, the transportation of goods by land is much more limited than transportation by water and exposed to accidents and/or robbery along the road. On the other hand, ships have always been able to carry pretty heavy cargo in a shorter time while cutting down on the price of the goods sold in foreign markets.

d) a constitutional amendment

it was the first amendment