The Marines were sent to the other side of the canal to provide security against any Iraqi soldiers or irregulars who might be hiding behind a large berm on the opposite side.
Obviuosly two Marines made it across and the two who drowned took a wrong turn and wound up under the water

Their training should have had them have sense enough to wear a saftey line. This will be one of many sad stories. I hope their deaths and the deaths of others are worth it.