It would be very difficult to climb out of the river, but would also depend on the tide. My dog often jumps in at high tide and can struggle to get out.
Yes, you can see the Claremont Ave houses from the park entrance, they back onto a track and then the park. It's very quiet there so you would hear screams.
You definitely could not just run into the pond. You have to go down a path, turn right and past a derelict building or across the field and through a small wood. It is a fishing pond so there are wooden areas where fishermen sit and there is a footpath around it. Personally I don't think it would be easy to stumble upon it in this huge park but in the dark you really can't see anything (I know this from my misspent youth, going to the same park with my friends to meet boys
)