If it is a solid tone, that is an overheat alarm and you may have a bad cylinder head temperature sensor. I suspect a bad sensor, rather than an overheat problem because you mention it did it right after you put it in the water, and it should not be able to overheat that quick. Do make sure that you are getting a good flow of water out of the tell tail when running though, and flush the engine on the hose for good measure. You have one in each head, and if you unplug one at a time when it is acting up you should be able to identify which one is giving you the problem and replace it. Do not just leave it unplugged, that tends to be dangerous and can make the engine run very rich.
If it is a series of three beeps it means the oil level is low in the engine mounted tank. If the tank is full either you have a bad connection, or a bad sensor. To check it unplug the two blue connectors going to it and check for continuity with a multimeter. When full you should have "NO" continuity, and when low continuity.
Aaron:cheers: