These are no practical use on a GRP hull, where you are trying to detect moisture inside the laminate. Unless of course you shove the points right through the hull. They are designed for wood and plaster.
They come in various versions with slightly different features (laser dot, adjustable emissivity, backlit display). Quite happy with mine. Very versatile - finding drafts, diagnosing the engine/fridge and checking the drinks are cold ;-)