Maximizing the battery life can be done in various ways. Lowering the screen brightness and timeout duration helps to save battery power. You can also enable the power save mode in the settings tab which will help in maximizing the battery life automatically.
The first step would be to close out all the apps that are open and not in use. Cleaning up the device storage space of unused files and applications will also help. If all else fails, factory reset the phone to restore it to default settings.
If you want to protect your phone from water damage in the future, you will need to purchase a waterproof case. Cases also exist that will protect the phone from invasive dust and drop damage. One example of a high quality water resistant case is the griffin survivor. Some examples of damage resistant cases are Evocel Dual layer Armor Protector and Tudia Ultra Slim Bumper Protective case.