Are engine hour numbers reliable?

Whenever someone is thinking of purchasing a used boat, one of the first things they consider is the number of engine hours it has. I've done a ton of work on my boat this season, and in the process accidentally disconnected the plug that connects the tachometer/hour meter to the ECM. The boat ran fine (I took it out not knowing of the disconnect), but obviously the tach and hour meter didn't function. So, my question is, is the number of engine hours stored in the ECM or the tach/hour meter gauge? I could figure it out myself by disconnecting it again and running the boat, but it's a pain to get to under the dashboard. If it's stored in the gauge, a guy could run hundreds of hours with it disconnected and then reconnect it and sell his boat as a low hour boat. And, even if its stored in the ECM, a guy could still run hundreds of hours, get a new ECM, and sell it as a low hour boat. I guess what I'm getting at is that perhaps one of the biggest factors in the value of a used boat - the number of engine hours it has - is pretty easy for a fraudulent seller to manipulate.