Honda 115 alarm

Motor alarm going off when boat is started goes off in 10 to 15 minutes of run time is this a bad sensor motor does not overheat

If it is a newer fuel  injected motor you should have a code stored in the engine control computer. Do you have a monitor gauge in the boat? If so it should tell you if it is overheat, oil pressure, ect.

It would hv to be the sensor... when you first start your engine it is cold. 

Certainly it could't be overheating on a cold start.

Alarm goes off as soon as I start the motor then goes off after some run time impeller replaced so I guess it the thermostat not operating properly is there anything I need to know before changing the thermostat

Huh?  That is the opposite of what I understood from you original post...

The alarm sounds when you start the engine, and goes silent after 10 minutes of run time - or the alarm is silent when you start the engine, but goes off after 10 minutes?

If the alarm is going off right away I'd suspect the sensor.  If it is the other way around, I'd suspect the impeller first, and then the thermostats.

Impellers should be replaced every couple of years.  Thermostats seldom go bad, but I have seen it happen a time or two.

I think the thermostat is likely the alarm starts with the engine then goes off after ten minutes

I'm with the boys on this one.  Alarm goes off in 10 - 15 minutes, it sounds legit.  Just because there is water coming out the "pee hole" doesn't mean it can't overheat.  If the impeller is warped or worn, it may not be moving enough water.  

The old saying goes "the most likely answer is probably the right one" and in this case, the most likely answer is the impeller needs to be replaced.

I find it odd that if the sensor was bad, it would take 10 or 15 minutes to alarm.

I would guess one of your thermostats is not opening, or your impeller is getting weak and / or missing a blade or two. 

I'd pull the thermostats out and put them in a pot of water on low.  They should open around 140 - it's very easy to see when they open.  Also, sometimes you may find some debris in there.  If this checks out I'd feel your heads while the engine overheat alarm is active.  I would estimate your outboard's normal operating temp is somewhere around 160.  If you have an infrared thermometer you could give it a quick scan to see.  Or if you have the thermocouple type, you could strap it to the head prior to the alarm going off. 

I'm guessing the alarm is legit...

Of course I'm just an HVAC guy who wrenches on his own stuff.  Boattech would likely have better info.

Was clogged at one time but shooting out fine now

going for the obvious... is it spitting water from the pee hole