I have one of these on my boat:
For two years it has worked flawlessly.
Some may say that is a lot of money, but if my boat sinks, probably won't be able to afford another of equally quality and rigged/equiped they way I want. In winter the harbor surge can be huge, would be nice to know my boat has broken lose and maybe getting holed by a piling before she sinks. You can hook to a smoke alarm to warn of fire danger, or to your hatch to warn of someone breaking into the boat- limitless possiblities. Good to have if I need to leave the boat for several weeks.