If you have Weldon #4, lay the skimmer on it's side and put some over the crack to see if it seeps in. Once it dries, put a little Weldon #16 over top of the crack and let it dry. DON'T USE SILICONE........silicone does not bond well with acrylic and it will be a disaster waiting to happen.
Oh, and don't try to smooth out the Weldon #16 after you put it on.......just put it over the crack and let it dry. It will dry smooth.