The boat is wealth. The person selling the boat gives some wealth away, in exchange of a monetary profit. That profit will enable him to buy wealth produced by some other person.
Let's make things clearly.
- There's the real economy. Here, work, capital, natural resources, etc are used to create wealth. People create their valuables and exchange them for things they value more, increasing the wealth of everybody.
- There's the nominal economy. Here money is invested in acquiring resources, that other people will spend money consuming.
In a simple theory, the real economy works alone and money is irrelevant. But that theory seems to be wrong. In fact, people gladly raise prices, but resist lowering them. That creates an asymmetry and the consequence is that the real economy works well¹ when the amount of circulating money is constant or increasing, but does not work well when it is reducing.
Now, finally, people getting loans makes the amount of circulating money to increase, and paying them back makes it reduce.
1 - At least according to the mainstream theories. Also, only up to a certain point.
Is that really how capitalism works, though? I believed it was more like this: making available loans at low interest rates stimulates the production of boats to large levels. The sale of these boats at profits creates wealth for the owner AND the employees of the boat making company. The employees spend that money and stimulate economic demand; the owners/investors make great profit and thus concentrate capital even more. This capital is then invested in other ventures, perhaps even as low interest loans to other consumers.
Loans/Debt enable businesses, consumers and nations to create economic activity NOW and accumulate the capital NOW; we don't know what the future will look like but the concentration of economic power (i.e. capitalism) has certainly played a most important part in technological progress.