It's pretty easy to beach a boat, if the river bank is fairly steep. All you do is charge the shore, create a wave, back off on the throttle, then follow the wave into shore.
Kerry's buddies on the boat seem to verify the story. This article (linked below)has a pretty good accounting of the event. It's not flattering, considering it's coming from the Boston Globe, especially the detailing of Kerry killing some civillians, and some friendlies, in a free-fire zone:
http://www.boston.com/globe/nation/packages/kerry/061603.shtml
Kerry initially hoped to continue his service at a relatively safe distance from most fighting, securing an assignment as "swift boat" skipper. While the 50-foot swift boats cruised the Vietnamese coast a little closer to the action than the Gridley had come, they were still considered relatively safe.
"I didn't really want to get involved in the war," Kerry said in a little-noticed contribution to a book of Vietnam reminiscences published in 1986. "When I signed up for the swift boats, they had very little to do with the war. They were engaged in coastal patrolling and that's what I thought I was going to be doing."