It's a sealed lead acid(SLA) battery which is different from a wet/flooded battery, which do require periodic topping off with water and ventilation.

For a project like this, SLA is the way to go.

https://electronics.stackexchange.com/questions/175649/are-s...

As canoebuilder said. I initially thought the same thing :). Believe me I googled the shit out of this so as not to drown the house in hydrogen.