바닥 is a bit of an umbrella term. It can be used for ocean floor, bottom of the river, or just bottom in general. 마루 is like a floor in a house. That said, I've never heard someone use 마루 in speaking.
In a house it is 마루 if one can sit or lie down.
For example, we do not say 화장실 마루 but 화장실 바닥 since we do not normally sit nor lie down on the toilet/bathroom floor.
Yeah no one just says 마루, and it would better be translated wood and not floor