Ablock has a volume of 0.09m3 and a density of 4,000kg/m3. what's the force of gravity acting on the block in water

       Density = (mass) / (volume)

                                4,000 kg/m³ = (mass) / (0.09 m³)

Multiply each side
by  0.09 m³ :           (4,000 kg/m³) x (0.09 m³) = mass

                                 mass = 360 kg .

Force of gravity = (mass) x (acceleration of gravity)

                           = (360 kg) x (9.8 m/s²)

                           = (360 x 9.8)  kg-m/s²

                           =   3,528 newtons .  

That's the force of gravity on this block, and it doesn't matter 
what else is around it.  It could be in a box on the shelf or at 
the bottom of a swimming pool . . . it's weight is 3,528 newtons 
(about 793.7 pounds). 

Now, it won't seem that heavy when it's in the water, because 
there's another force acting on it in the upward direction, against 
gravity.  That's the buoyant force due to the displaced water.

The block is displacing 0.09 m³ of water.  Water has 1,000 kg of 
mass in a m³, so the block displaces 90 kg of water.  The weight 
of that water is  (90) x (9.8) = 882 newtons (about 198.4 pounds), 
and that force tries to hold the block up, against gravity.

So while it's in the water, the block seems to weigh

       (3,528  -  882) = 2,646 newtons  (about 595.2 pounds) . 

But again ... it's not correct to call that the "force of gravity acting 
on the block in water".  The force of gravity doesn't change, but 
there's another force, working against gravity, in the water.