Basement Watchdog Sump Pump High Water Alarm on our boats for the better part of 15 years. Never had a single failure and they simply work. They are VERY loud and you can hear them clearly even with the engine running. They cost under $10.00 each and run on a 9V battery. I replace the batteries yearly and that is all that's necessary. To test just get the two contacts wet.. We have three on our boat and love them. One of them even made the move with us from two boats ago...

The one for the engine compartment can bee seen in the upper right of the picture. The white box with red writing.. The sense wire runs down into the engine sump..