Does the trailer have a tire pressure sticker, or pressures listed in the owner's manual? I'd go with that pressure unless your boat is near or above the limit of the trailer.
Depending on the tire manufacturer, you may be able to call a help line, describe the tires, trailer, and load, and they will tell you what pressures to run. I did that on my truck with BFG.
I never assume that the tires store set the pressures correctly, even when I tell them up front what pressures I want. Although Discount Tire is usually better than most.
All of them being at 65 means to me that it was done on purpose. That doesn't mean it was done *right*.
I went through some of this trying to figure out what pressure to use on my wife's horse trailer.