If you're only in shallow water, line stretch isn't as much of a factor (one of the reasons a lot of guys use the heavier line), so you can use lighter line. I'd say 20#. That'll be enough to pull the halibut off the bottom, but not too much so that it effects the bait, or spooks fish (hopefully). Basically, the more line you have out, the more it's going to stretch. Heavier line doesn't stretch as much, so for deeper water where you have a lot of line out, you want heavier line to cut down on line stretch. But when you're up shallow, there's not much line out, so it's not as much of a concern. Some folks even use spectra in the deep water, which has virtually NO stretch.
-Brian