The best option is a security cylinder with a protected key, the kind where copies can only be ordered with its card. That way nobody can have a copy made on their own during their stay.
A holiday let has lots of people through it each year and keys come and go. It is normal for one to get lost or not come back. With a protected key you know how many there are, and if one is missing, the cylinder is changed and that is that.
Other useful ideas:
- If the house has several doors, key the cylinders alike so one key does the lot.
- Choose cylinders that stand up well to salt air, especially on the seafront.
- Check the lock between seasons, which is when it gets the most use.
It is explained in holiday lets.