1990 40hp evinrude alarm went off

[Joel Kiss]

I pulled into the my boat slip after about 15 minutes of running the engine and the alarm sounded the water guage hadn't moved either

 

[joereeves]

There are three (3) different horn warnings... you don't say which one you encountered.

Beep every other second = VRO failure (no oil)
Beep every 20 or 40 seconds = Oil tank down to 1/4 tank level.
Beep constant and steady = Overheating

I'll assume overheating.... either your lower unit grabbed onto a plastic bag or something to restrict the water flow temporarily OR the water pump is failing.

 

[Opti-man-808]

Stuck thermostat to add to joereeves list if overheat