It's impossible to say what type of rock you'll find at the bottom of a body of water without more information. The type of rock depends on many factors, including:

* The geological history of the area: Different areas have different bedrock formations.

* The type of water body: Lakes, rivers, oceans, and seas all have different geological processes that affect the types of rocks found on their bottoms.

* The depth of the water: Deeper waters are more likely to have rocks eroded from surrounding land, while shallower waters may have rocks deposited by rivers or glaciers.

* The presence of sediment: Sediments like sand, mud, and silt can cover the underlying rocks.

Some common types of rocks found at the bottom of bodies of water include:

* Sedimentary rocks: These are formed from the accumulation and cementation of sediments. Examples include sandstone, limestone, and shale.

* Igneous rocks: These are formed from the cooling and solidification of molten rock. Examples include basalt, granite, and obsidian.

* Metamorphic rocks: These are formed from existing rocks that have been changed by heat, pressure, or chemical reactions. Examples include marble, slate, and gneiss.

To give you a more specific answer, tell me more about the body of water you're interested in!