you have to fill an empty bottle with water and then water one of the plants and the bomb will frow then you throw it on the rock and it explodes and water comes out.

learn the wind requrium thingy. Go outside the entrance to the post office mountain and
below, you should see a shrine. Drop down and press 'A' to one of the shrines. A wind god
will appear and grant you the power of controlling the direction of the wind.

Once inside, use the power to blow north, unfortunately you need a dead hit for the rock
to blow up. A cut scene will appear, in which the area is flooded.
Swim across, and that's all thee is to it!

ok, first, go learn Wind's Requim(or whatever it's called) from the gravestone thru the
cave when you first reach the island, then go to the area, change the wind diretion to
North, and fling the girl over the cliff to get an empty bottle, now fill it up witrh
water, climb back up and water 1 of the plants and blow the rock sky high