I don't like the #2 and #4. As for the prepositions, it depends what you mean. I wouldn't say "in the pond"... they went to a stream, right? Then it depends where the steam is: it can be beside the pond, near it, etc. Do you know what prepositions mean? If you do, then it's the same difference as between "We usually meet beside the bank" and "We usually meet at the bank".