If you necessarily need to run both operating systems bare metal and want/need to share the data between these OSes, you will need a partition with a filesystem that both OSes understand. The lowest common denominator is FAT (FAT32), NTFS/exFAT are also an option (also see exFAT vs NTFS on Linux). On top of that you may want to have a separate partition for your Linux installation -- one that will have the benefits of an advanced filesystem (XFS, JFS, ext4, ZFS, Btrfs - you name them).
The answer thus is: no, you don't have to partition it, but you probably want to. It is common to "partition" a drive even when it is holding just one partition anyway, since that allows the operating systems to recognise that probably there is some data there that should not be discarded.
That said, it may be more practical to choose one of the OSes as "primary" and run the one of virtualised (since you came here, maybe running Windows in the VM under Linux). Then you can just pick a directory (or directories) that your host system is going to export into the virtual machine - this would typically be done via the SMB networking protocol or some other mean (for example VirtualBox has it's own specific way of providing filesystem data exchange between the host and guest operating systems).
One of the advantages of having an OS virtualised is that rollbacks of the OS itself are much easier. Which, from my personal experience with OSes that lack reasonable text configuration, can be a sanity-saver indeed. Another one -- if you are so inclined -- is encryption: full-disk encryption (including the shared partition) will be a bit more complicated to set up. On the other hand, there are of course things that will be more complicated in the VM - using GPU fully may present some challenges for example (depending on your particular hardware setup).