Not sure what to tell you about waterproofing. Potting and encapsulants tend to be rather pricey.
As for the LED being able to be hooked directly up to a 12V battery... my guess is that the LED's forward voltage is roughly 12V... i.e. it just starts to conduct somewhere around 12V. As you note, if you are running the light for 6 hours from a 8Ahr battery, then you sure aren't drawing 2.5A. I would guess it is closer to 1 ampere. It might be fun to connect a meter in series with the LED and measure the current.
In general, it's nice to have something that will limit the current to a safe level over the full range of the applied voltage. Assuming that you are using a lead-acid battery, they will generally be 13.5VDC or so when fully charged, and can run down to .... 11V or 10V? You might benefit from adding a resistor in series with the LED in order to keep it safe with a fully charged battery. A rough estimate would be 0.5 ohms, and rated for at least 3 watts.
If you have any questions on customized COB LED. We will give the professional answers to your questions.