Besides other uses (for comparisons), como is employed to mark conditions and causes. The meaning depends on context and above all on the tense and mood of the verbs.
In your example the answer would be
Como te alejes de la orilla, no te traigo mañana.
As you already deduced, this is a conditional; but in this case, with como, you need to use the subjunctive mood and say alejes, instead of the indicative alejas (which you would use if the conditional phrase began with si). This kind of sentence is rather common with threats:
Como te vea otra vez por aquí, llamaré a la policía.
You use the subjunctive ([tú te] alejes, [yo te] vea) because the situation is hypothetical.
You will also find como introducing a cause. In this case the verb will be in the indicative. You can have a perfectly parallel phrase to the one above:
Como te alejas de la orilla, no te traigo mañana.
This means: "Since you're getting away from the shore, I won't bring you tomorrow." It's grammatically correct, but the actual situation doesn't look plausible, so I guess the answer should be the other one.