that impeller looks fine, you are either losing suction on the intake side or the bellows are plugged. Another possibility but unlikely is the water pump being bad.
FYI using a garden hose to trouble shoot this is not recommended because of the fact that it is pressurized. Also If you do for whatever reason hook a hose to the boat to run it only run it at idle, I have seen many times guys revving the engine and actually sucking the hose dry and collapsing it.
Go to big lots buy a 55 gallon tote for $15 and use that to run your boat in the driveway, I can tell by the housing and impeller that you have a Volvo Penta out drive, more then likely an SX. If this is the case then the tote will work perfect for you.