Have 2 SP120 panels that, according to my multi meter and inability to charge AC200 as they once did, are now dead. Currently involved in emails with service. Did you ever find out what killed your panels ?
IP68 is fully waterproof and a quick google search reveals :
With an IP65 rating, the LEDs can be used in an outside setting and are water-resistant but they are not waterproof and are not suitable to be submerged. An IP68 can be submerged in water.