They probably did that because their children would also be sold as slaves, so they didn't want that for their babies. They chose less evil in the time of need. That's also why the monster probably reacted to the boat rather than the man himself. Because it recognized the one thing that they assimilate with their mother.
OH! So it's the collective conscious of the babies that died from long ago...that's sad actually :(
It is very sad. Also, they mentioned that the women did that because they knew Isis would save them, which she couldn't because she didn't want to use her magic and make Seth spot her, but those women didn't know abt this. So they didn't try to kill their babies, they just wanted to give them to Isis and it is so sad that they unintentionally killed them and probably died themselves without knowing :(
They didn't really know that tho. It's wasn't about is having a streak of always saving drowning babies. It was about faith.
Wait, what?! The women did that!?