What happens when ship enters sea from river?

Step 1:Formula of buoyant force

The density of seawater is more than river water

buoyancy is given by the formula $F_b=V_s\times \rho \times g$

where

$$\begin{gathered} F_b-\mathrm{buoyant}\mathrm{force} \\ {V}_s-\mathrm{submerged}\mathrm{volume}\mathrm{of}\mathrm{the}\mathrm{ship} \\ g-\mathrm{accelaration}\mathrm{due}\mathrm{to}\mathrm{gravity},\rho -\mathrm{density}\mathrm{of}\mathrm{the}\mathrm{fluid} \end{gathered}$$

Step-2:Conclusion

1. A ship experiences more buoyant force in seawater as compared to river water, volume of the ship immersed inside water decreases to water
2. This decrease in ship's volume is due to the balancing of its weight

Hence it rises when it passes from river to sea.