Maletrain: As sd45elect2000 hinted at, the real reason for not running diesels through water is that the traction motors REALLY don't like it. In the early 70s I heard, on the BN, that there was only about 4 inches clearance between the tops of the rails and the bottom of the motors.
Of course, that wasn't an issue in most of eastern Washington, where enough water to cover the tie plates was extremely rare.
Steam locos don't much care, as long as the firebox is above water. Just make sure the track is good!