Final answer:
The American colonists of the 1600s settled along the seacoast and rivers primarily to engage in trans-Atlantic trade as per mercantilist policies, which allowed them to export raw materials and improve their economic standing.
Step-by-step explanation:
A major reason the American colonists of the 1600s chose to settle along the seacoast and rivers was to participate in the trans-Atlantic trade. The mercantilist system emphasized colonies as sources of raw materials to fuel the growth of the home country, which in this case was England. Colonists obeyed trade laws like the Navigation Acts because they were profitable and allowed them to export their goods (though only to English merchants or English colonies). Settling along the seacoast and rivers made it easier to ship these raw materials out and receive imports. They also wanted to improve their economic positions by participating in the burgeoning consumer revolution happening in Europe.
Using waterways for trade was integral for the economic success of the colonies, more than for other purposes like providing defense from Native Americans, using water power for machinery, or for exporting slaves. The early colonists in British North America primarily engaged in farming rather than industrial manufacturing. The geographic choice of location for settlement played a crucial role in enabling trade and ensuring the colonists' ability to participate in the global economy of the time.