Hasegawa makes some, including a kit that can be completed as either a VIIC or a IXC - don't know how that would work out.
They are part of the waterline series (which means they can be placed on a flat surface - because of a lack of a keel) and actually include a sinking freighter, apparently.
I have a Hasegawa Akagi, which I think is alright quality-wise (although I am an amature modeler) and also a Kaga kit, which I haven't built yet - I don't know what their U-Boat kits are like.