Shifting to the Top view, hit [A] to select the entire model, then [W] to bring up the subdivision dialog. Choose Subdivide.
Now comes the fun part. Select the left half of the model using Box selection: hit [B], then left-click and drag a marquee over the model to select a part of it.
Delete the faces by hitting [X]. Don’t worry that you’ve removed half the model: all will soon become clear…
Hit [Space] to bring up the dialog. Type ‘Add Modifier’ into the text field to bring up the Add Modifier dialog. Choose the Mirror modifier.
Voilà! The missing half of the model reappears. Mirroring the model like this removes the slight asymmetries that are introduced when modelling by hand.
The model is starting to resemble a whale, but it looks more like a toy than a real animal. You need to refine its shape.
Start adding loops that run from the head of the whale to its tail. Hit [Ctrl]+[R] for a loop cut, left-click on the model to place and move it, then left-click again to accept the position.
The image above shows the position of the first loop; the one below shows the second.
Now start adjusting the loops to alter the cross-section of the model. Begin with the lower of the two loops you just added…
…and then adjust the upper one. The model should begin to look more organic.