Constant Alarm - 01 Merc. XR6 150

[sefaber35401]

First time boat owner....bought a 2001 Procraft Pro185, w/ Mercury XR6 150. No problems thus far, until yesterday. On my way into marina, I saw a young couple stranded in a small aluminum boat. Offered to tow them in....about 1/2 to 3/4 of a mile to the dock. I pulled them at a slow speed, but obviously was not able to get on plane (3200-3500 rpm roughly). About 300yds from the dock, a CONSTANT alarm began to sound. I immediately slowed down to just above idle speed and in about 30 seconds the alarm stopped. we coasted into the dock and all was well. I loaded the boat without running after the incident, however the alarm never returned in the 2-3 minutes running/idling before I put on trailer. Couple questions;

What was the alarm?
Should I have it checked, pre-my next trip?
Any checks/trouble shooting I could do myself?

Thanks!