I once found a stream with a rocky bed that was just below the waters surface. This let me walk way out into the stream with my boots keeping me high and dry. It was a neat experience walking "on" the water.
Is it the "pe" that affects this sentence so that the "apă" is indefinite where we would use definite in English? Or does it have to do with the verb? Or is it random phrases I need to memorize? (Sorry if this question sounds vague)