Leaving Caras Galadhon, The Fellowship jumps in a couple of boats and heads downriver towards the Gates of Argonath. At this point, you can walk next to the river all the way, or you can use a mod to get the classic Minecraft rowboat and float down instead. I recommend the rowboat because reaching the two giant monuments as you enter Gondor is a spectacular view from the water. It's how the incredible sight was meant to be seen.