Do not try to defend the water immediately with shooters. It is too expensive early on.
Note: This requires you to have unlocked Fume-shrooms from an earlier level, which you have. plants vs zombies 4-1
Puff-shrooms are free, but they disappear after 60 seconds. If you build your entire front line out of Puff-shrooms, you will suddenly have an empty lawn when the final wave hits. Do not try to defend the water immediately with shooters