Ben,
There's supposed to be a square-profile o-ring seal in a groove on the water pump flange. No sealant is required, although some like to lightly coat the o-ring with a film of silicone grease to help it stick in the groove during installation.
If you've got a bad leak, it's possible the o-ring seal fell out of the groove during installation and got pinched.
Good luck,
John L.