Take a look at the original sentence again:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns, Bo was pretty sure that sticking
the huge fruit under his shirt and pretending that he was pregnant wouldn't
fool the cashiers.
You chose to fix it the correct way:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns. Bo was pretty sure that sticking
the huge fruit under his shirt and pretending that he was pregnant wouldn't
fool the cashiers.
He had money only for hot dogs and buns is a main clause. Bo was pretty sure is the second main clause. You are always correct when you use a period and a capital letter between the two main clauses that create the comma splice.