Not DDM and not a doctor, so feel free to tell me to shove these thoughts wherever you please:
The challenge with piercing-related infections in the ear cartilage is that it is an EXCEPTIONALLY low-vascularized part of the body. There aren't very many blood vessels and not much circulation. Once an infection takes hold, it will be very difficult to treat and heal.
In very severe cases, the tissue can become necrotic (i.e., dead). This can require surgical removal of the necrotic tissue, which - again - will be hard to heal. Some people end up with ear prostheses, others may have trouble with glasses/hearing aids/headphones for the rest of their life.
It's certainly possible that nothing could go wrong. However, the consequences of something going wrong could be not only serious, but disfiguring.
Personally, I would not take the risk.
Basically true but just one clarification. The pinna itself (the part of the ear that sticks out) has excellent vascularization of the soft tissue (skin and fat), and that part tends to heal well from surgical incisions, etc.. But the cartilage (the stiff part of the pinna that does not include the earlobe) gets its blood supply from the perichondrium, which is a layer that surrounds the cartilage itself. Just like in the nose, anything that interrupts the supply from the perichondrium can cause death of the underlying cartilage, with deformity as the dead tissue fibroses (becomes stiff with scar tissue). This is the way that a "cauliflower" ear forms. Trauma (classically from boxing or wrestling) cause a hematoma (a collection of blood) under the perichondrium, interrupting the blood supply to the cartilage. This can also happen with the nasal septum, and draining a hematoma like this is therefore an emergency.
With cartilage piercings, the issue isn't so much blood supply but contamination of cartilage, which can cause infections that can be difficult to treat. Usually, after a few months, the piercing tract has healed over, so the cartilage is no longer exposed to the outside world. But you can definitely have complications like this, with permanent deformity of the ear.
If there is any sign of significant infection, topical antibiotics may not be enough. The treatment is removal of the piercing and oral antibiotics may be required. Generally, for patients over 18, we use fluoroquinolone drugs like Levaquin, since they are one of the few drugs that treat pseudomonas bacteria (a common external ear organism). If you go, maybe get that at home in case you are going someone with no access to medical care, along with a topical ointment.
There really isn't any reliable way of keeping the ear completely dry when diving. As DDM said, I wouldn't rely on anything for that. If anything, a hood or pro-ear mask might make things worse by rubbing against the fresh piercing.