What insurance covers a boat trailer accident when attached to a car the car insurance or the boat insurance?

If you only have liability your car insurance isn't covering your boat trailer, even full car coverage probably has small fine print limiting coverage to your car. Trailer coverage through your boat insurance provider is more likely to cover it.

Additional answer

If your question is regarding the damage caused by the trailer while it was being towed by an automobile, then the answer is yes, the damage caused by the trailer will be covered by the liability. The damage to the trailer would not be covered by the auto policy even if you have collision coverage.

If it is regarding the damage to the trailer, it would not. Only if you have a watercraft policy which typically provides coverage for both trailer and watercraft.