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I have never heard of brakes for a 15' boat trailer, yet alone electric brakes for a 15' boat. Sounds like a total waste of money to me and bad information from a dealer.

A 150 Montauk trailer does not need brakes at all, and electric brakes are generally not recommended for boat trailers in general.

All boat trailers use hydraulic brakes, and if you are using disc style brakes, really the only workable option, a 5 wire flat plug is required for the back up light circuit, which blocks brake fluid so the brakes don't lock up when in reverse.