Not any use for GRP hulls - designed for timber or plaster.
More on the meter - the Aldi one is designed with two sharp steel prongs you stick into the wood or plaster, apart from damaging the hull it will give very unreliable readings on GRP. There is a reason moisture meters for hulls cost £200+ and the Aldi one £9.99.Most moisture meters are calibrated for timber. Make sure you use the relative scale and it's just that, relative. Easiest way is to find a boat that is known to be the moisture content your after, try your meter on that and then use that as your target.
More on the meter - the Aldi one is designed with two sharp steel prongs you stick into the wood or plaster, apart from damaging the hull it will give very unreliable readings on GRP. There is a reason moisture meters for hulls cost £200+ and the Aldi one £9.99.