A boat in the river enters into the sea water, then it                                           

Actually a boat in river would have displaced some water equal to its weight.

Weight of  boat = weight of river water displaced = ${\rho }_{\text{river}}{\text{v}}_{\text{immersed}}\text{g}$ .

When it enters into sea water again its weight is equal to weight of sea Water displaced Weight of boat = weight of sea water displaced $={\rho }_{\text{seawater}}{\text{v}}_{\text{immersed}}\text{g}$  

            We know

${\rho }_{\text{seawater}}>{\rho }_{\text{river}\text{water}}$

So it will immerse in sea water less volume than in river water.

Therefore we get a boat in river water after entering into sea water will rise up.