Underwater – LED or Incandescent only. 175w pulse-start lamps outlawed, so no HID or fluorescent underwater – in-rush current made it dangerous. 10,000 volts to start it up made it super unsafe. Restricted by UL to no more than 150v. ABSOLUTELY WILL NOT DO IT, even internationally. 12 volt predominantly sold overseas.
12-volt typically used for swimming pools, etc. Florida’s regulation only allows that. Set by state codes.
Underwater – voltage drop. Underwater transformers are 12, 13, or 14 volt. Need 12 volts at the fixture, some drops in transit. Thicker cable prevents voltage drop. 10/12 gauge cable to J-box from transformer, 16 gauge to fixtures from j-box. Lower voltage at fixture causes a dimmer fixture. Once it’s down below 10 volts, the dimming becomes visible to naked eye. This occurs when someone goes beyond the recommended maximum cord length listed on spec sheet for the fixture. This is relevant in a few places but it’s most common in the underwater world. This is entirely due to length and gauge of cord.