Nice story, cartman. Simple examples help.
Note a few things:
 
-There's no money in our example and that's why "interest" can go to zero. In fact, one could say that there's no such interest but a sharing of profits.
The basic interest of money doesn't ever goes to zero, no matter how little the growth is. Interest stays there even within a recession.
-For some reason, cleverguy prefers to pay 100% "interest" instead of saving the fish himself.
-Since there's no surplus, lenders must hunger and that may be a reason why they reclaim interest. If there were surplus of production, people could lend fish at no interest because their fish would rot otherwise.
-Within capitalism, the nets would need to yield at least as much as "money does", taking it first from profits and then from wages. The production of nets would have stop when that yield is compromised (until some nets break or the tribe gets bigger) instead of when the demand is met. In your example, there's free market but there's no capitalism.

But in general, yes, it seems that the liquidity premium gets higher when there's very profitable investments to make.
On the other hand, after the initial profits move to wages, people can save more, decreasing the basic interest.