When a ship moves from freshwater to seawater, it will experience a change in buoyancy due to the difference in water density. Freshwater has a lower salt content and therefore a lower density compared to seawater.

Option 1: Sink completely - This option is incorrect because the ship will not sink completely. Even though seawater is denser than freshwater, ships are designed to be buoyant and can displace a significant amount of water to stay afloat.

Option 2: Sink a little bit - This option is incorrect because the ship will not sink when moving from freshwater to seawater. Instead, it will experience a change in buoyancy.

Option 3: Rise a little higher - This option is correct. When a ship moves from freshwater to seawater, the increase in water density will cause the ship to rise a little higher in the water. This is because the denser seawater provides more buoyant force to support the ship.

Option 4: Remain unaffected - This option is incorrect because there will be an effect on the ship when it moves from freshwater to seawater. The change in water density will impact the ship`s buoyancy.

Therefore, option 3 is the correct answer.