Originally Posted by Polwart
There a kit much discussed on here which costs less and can do what you want. I think? it was Leapy who started the thread if you are searching...
Quite right. There is the "original" eBay product from a Chinese company called Xexun I think, then there are various clones of that product. Prices seem to be in the sub 100 pounds area.
There is also the UK-designed Yacht Sentinel, which sells for more like 300 quid but tells you about the status as well as the location of your boat: If your batteries are going flat, or if there is a frost/fire risk, if there is any water building up inside, etc etc.