does buzzer always mean top end issue?

My first trip in mid feb. my alarm on 70 yamaha sounded.

I would like to know if the issue can be caused by the water pump or is it a top end issue only.

My oil reservor was full, and I checked the lower unit the greese was ok. if the idiot lights are all working would you think if the engine wasnt getting oil they should be going from yellow to red?

I stuck it back in and no more horn or siren, boat had been stored mid dec.

I was lucky the warning siren was short lived and lots of hours running since this incident.

could be whats called a short pick up. if stored for an extended period the oil will settle and not circ untill ran. if you shut it down and then started it back and it didnt happen again this is more than likely what happened.