In his after action report, Kerry feared the consequences should he say he struck the fishing weir with his boat so he blamed the damage to his boat as being from a near miss from a mine.
And there's still the third variation, told by Kerry himself and entered into the Congressional record, in which he says Rassmann fell overboard when Kerry initiated a sharp turn to starboard - no mention of either a mine or an object.
http://www.johnkerry.com/pressroom/releases/pr_2004_0117d.html <-- Kerry's 2004 version
On March 13, 1969, Rassmann, a Green Beret, was traveling down the Bay Hap river in a boat behind Kerry's when both were ambushed by exploding land mines and enemy fire coming from the shore. Kerry was hit in the arm, while a mine blew Rassmann's boat out of the water. With enemy fire coming from both sides of the river and swift boats evacuating from the area, Kerry's crew chose to turn their boat toward the ambush to save Rassmann.
"We were still under fire, and he was wounded at the time...," recalled Rassmann. And with his boat's gunners providing suppressing fire, Kerry extended his wounded arm into the water and the two lieutenants locked arms.