The only way to know how much tongue weight you have is to weigh the loaded trailer and then weigh the tongue. I have seen recommendations from 5 to 15% of the total trailer weight. Take it to a truck stop scale, drop the boat and weigh the truck, pick up the boat and weigh it again. See if you can get all the axles on different scale pads
Our new to use "stink potter" and trailer weighs about 5k and the tongue weight at the tow height is about 75 pounds. Just over 1%. This tows like a dream, we have to look back and make sure it is still there. I can pick up the tongue about an inch, then it gets real heavy.
I do not recommend a 1% tongue weight. If anybody lives nearby I have a trailer tongue scale.
The sidewalls on a trailer tire are different than a regular tire. When you turn a dual axle trailer sharply the tires will slide sideways and put a real strain on the sidewalls. Go to an empty parking lot and make a sharp turn and watch in your mirror.
Xort is right about the WD and sway bars. I don't think you could put it on a boat trailer anyway.
When you check the springs, one or more might be cracked in the middle, right over the axle. Might be hard to see.
I was just checking around and "C" rated tires are 2150 pounds max each. 4 x 2150 = 8600. "D" tires are 2540. 4 x 2540 = 10160
36' and dual axles with that much weight might be the problem. I recommend weighing the trailer loaded before going any further. You might just need to add an axle to the trailer.