Easiest thing would be that the inlet is clogged with weeds or other debris. Take a good look around the inlet (it will be near the impeller) to see if it is blocked. If you are not getting flow while its on the hose, then you may have a blockage in the cooling system, which could be a lot harder to locate and fix.
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100