problems with water intake

Ok so here is the deal. Boat is running but isn't taking in water like it should. Here is what I've done....and forgive me if I'm using the wrong terms for some stuff. At the water filter water is coming in fine free flowing without the boat on....right after the filter the is what looks like a pump even though I don't think it is if flow freely through that. then after that there is a long hose that takes the water up to the impeller.....it never makes it to the impeller. So I removed the house after the filter that is going to the impeller and put the garden hose directly into that and turned the boat on and the water comes shooting back out towards the filter never making it to the impeller. so i removed the hose on the discharge line of the impeller to make sure it was turning and it was but still no water. next i took the impeller out and there was nothing that looked real wrong with it but I've never even opened one up so I don't know how it should fit or anything like that. I had it replaced by a marinia last winter. The only markings on it was "16". The boat is a 1987 Supra with a PCM engine.