Review the original item:
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 would not fool the cashiers.
You wanted to fix it this way:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns,
but Bo was pretty sure that sticking the huge fruit under his shirt
and pretending that he was pregnant would not fool the cashiers.
A comma with a coordinating conjunction like but is an appropriate method for fixing a comma splice. However, when you use this method, you must choose a coordinating conjunction that makes sense in the sentence.
But shows contrast, and in the example above, a contrast transition is inappropriate.
You might want to consult the rules for fixing comma splices and fused sentences.