Both of my raw water strainers work on a suction principle. The are mounted above the water line. With the top off, they would not work at all. Once the water already in the strainer drained out, the engine would be dry.
Is yours mounted below the waterline?.
Yes it is. In fact, when I do it, I have to control the flow with the intake valve. A while back the same thing happened and someone suggested taking the top off and it worked. Not sure why, but I think it reduces the surface tension in the rest of the line. Doesn't really make sense to me because I would think the water pressure from the intake would offset any surface tension. Anyway, it didn't work this time!