Always start from land. If the drone loses contact, it is common for it to return to the starting point. If the starting point is on a boat that has moved, the drone will likely land in the water.
Do not fly over water. If the drone has a problem, it is common for it to go down for landing. If it is above water, it will then land in the water.
The electronics in the drone are sensitive to knocks, so be very careful not to drop the bag it is in or hit it.
Check with the owner which software to use.
Check where you can fly and not.
There are many good model-specific instructions on eg Youtube, check these out before you fly.
Remember that when renting a drone you are responsible to pay a deductible of 5000 kr in the case of damage or loss.