Hi! This is a guide on how to move a cart across a river/sea.

All you need for this is to take the cart into the water and jump on its back so that it leans in the same way as in the screenshot.
Your character will stand on it, and at the same time will not sink. And all you have to do is just keep going. You will also have the opportunity to steer the cart.

Taking the cart across the river this way is a little faster than even rafting across the river.