A boat with an 8' (96") beam is considered by most states to be the maximum trailerable width without applying for a special permit. (In some states, said permits are one-time only deals and you have to apply for each transit.) Most trailer boats from 21' and up have an 8' beam.
Sooo... If you have a boat with an 8' beam, it is obvious that you need clearance lights on the trailer. It makes good common sense, too, so folks merging behind you realize that you are towing.
As an aside, can we talk trailer brakes? Surge brakes are so unsafe. The mechanism usually corrodes, so they either don't work, or the brakes get stuck on. Splurge and buy an electric brake system tapped into your car's brake system. The trailer brake will modulate with brake pedal pressure. If the trailer ever gets unruly, you can reach down to a control box and manually apply trailer braking to straighten the rig out.