The detail for lakes depends on the location.
In some areas, you will find during "Step 2" of making a custom map, there is a checkbox option for Lake Depth Contours. Checking this will superimpose bathymetric lines on the lakes and rivers (where available).
In some cases, the land was surveyed before a reservoir was put in and the underwater contours remain.
Be sure to use the Preview Now feature when creating a custom map to see exactly what will be included on your printed map.
Also, we invite you to check out our pre-made Lake Depth Maps at: https://www.mytopo.com/products/lake-maps.cfm