The main difference between the flotation of a ship in a river and the flotation of a ship in the sea is the level of buoyancy. In a river, there is less water to displace and therefore the ship will sit higher in the water. This can be seen when boats are moored in rivers – they often have their gangplanks at a lower level than when they are moored in harbours.
The second difference is wave action. In open water, waves will act on a ship from all directions and will cause it to roll. This rolling motion helps to keep the ship afloat by keeping water from entering through any cracks or openings. In contrast, when a ship is floating in a river, there is only one direction of wave action (from the flow of the river) and this can actually make it more difficult for a boat to stay afloat.