A scuba cache is an underwater cache that is deep enough to require scuba gear, and certification for recreational scuba diving. They are not intended for everyone, and they should have a terrain rating of 5 to reflect the specialized equipment and training required.
If you are seeking a scuba cache, be prepared and have all your gear in good condition, tested as necessary, properly packed, and ready to travel. You don't want to forfeit a scuba cache because you forgot to put your mask back in your bag before you put it in the car.
After you have found the scuba cache, make sure to re-seal it carefully, to make sure it remains water-tight.
If you are creating a scuba cache, make sure your cache description indicates any and all equipment AND any special certifications required for the dive. (Don't send a cache hunter into an underwater cave or wreck site looking for a cache if they are not cave- or wreck- dive certified.)
Make sure the container is durable and easy to unseal and re-seal as the more complicated the device, the less likely it will maintain its water-tight integrity. Hunting for a scuba cache and getting it to the surface can be disappointing if its been flooded due to faulty equipment or a negligent cacher.