My '81 has a 3/4" id hose from the seacock to the head but it rises well above the waterline under the v-berth, then runs down to the head. I think this is functionally the same as a loop.
There is an in-line check valve near the high point.
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI