With the boat on the trailer, you can't tell they might need to be balanced. Pull the trailer down the road without the boat on it and I'll bet you'd notice.
I have a 10' Utility trailer that I pull my Zero Turn mower on, and without the mower, it was vibrating the crap out of the trailer and I could even feel it in the truck. Tire place I took it to said the same thing..."we don't usually balance trailer tires". These were 15 inch tires/wheels. I said balance them anyway. Night and day difference pulling the trailer, particularly unloaded. Tires/wheels do exist that do not need balancing, but they are as rare as hen's teeth. Just the valve stem alone can throw the balance off.