'89 Waverunner 550 is not cooling

[OK Toon]

We've got an older Yamaha Waverunner with the 550 engine. The motor runs great, but it is not getting any water through the cooling hose. So once the motor gets hot, the sensor shuts the motor down.

Any clue where to start looking? The hose is not clogged -- we've taken it off and inspected it.

Thanks for any assistance.

Sorry, that should have been a Waverunner 500.