If additional buoyancy is required for a part, then Point Buoyancy elements may be included.
To add a Point Buoyancy element:
Select a Part in the tree view.
Right-click the Part and select Add > Point Buoyancy, or click the Point Buoyancy icon in the Parts toolbar.
Select the Point Buoyancy object in the tree and set the properties. Point Buoyancy objects require a position (X, Y, Z) and a Volume.