Gasket or O-Rig
I'm not sure what brand you have, but the Jabsco Y-valve (and others?) uses an O-ring. It is possible that it is not properly seated. You can easily open it, clean and dry the ring. Make sure you use a silicon bases lubricant, then re-assemble. (Note: Not a silicon glue or adhesive.) Vaseline or other petroleum based lubricants will cause deterioration of the rubber O-ring. When reassembling, be careful that the ring stays in the groove, and don't ovetighten. It really is an easy job and shouldn't take more than about 30 minutes. Oh, and don't forget to close the thru-hulls first. As to your idea of just putting sealant on the outside, it probably won't work, or won't work for long!
