Rabbi Yehuda says that a shlulit is a place where rain water gathers.
Rav Bibi Says that Rav Yochanan Said that it is a water channel.
for the one that says that a "puddle' is enough, OF COURSE a water channel divides a field.
but if you agree with Rabbi Yehuda, who says that a water channel is enough, than you would definitely not agree with a puddle, as it is not nearly big enough. The Gemara agrees with this opinion.