On this page of the Sons of the Forest guide, you will find the location of the shovel. You will learn what steps you need to follow to obtain it.
Finding the shovel is not an easy task, as it requires visiting as many as three different caves scattered throughout the map. In the first two you will find equipment that will help you to get to the third cave - containing the item you are looking for. This will take you some time.
Where is the shovel?
To reach the shovel, you'll need two things - a diving kit and a line gun. You can find both of these things in caves. This means you will have to go underground.
You can find the diving kit in the place marked in the picture above. You can also find it on our map at gamepressure.com: Cave #2 This complex is relatively undeveloped, so you won't have any problems with finding the item you're looking for. However, you must bear in mind that the area is patrolled by a shark.
After leaving this location, you should go to the cave marked on the map above (Cave #1). There you will find a line gun.
Once inside, you have to blow up the creature blocking the passage inside. You can do it with a C4 lying nearby or with a grenade (you will unlock the PINATA achievement). The line gun is in the box at the very end of the location. You need it to leave this place, so you won't miss it.
Finally, you can go to the cave with the shovel (map link: Cave #4). It is located in the place marked on the map. Just like the line gun, you will find this equipment at the very end of the complex. It's lying next to a corpse, so it's hard to miss.