Re: long distance boat haul
1. I would check those tires not only on the boat but your tow vehicle. Check the air pressure. If your boat tires have any signs of weather rot or low tread it wouldn't hurt to replace them. If it was me going on that long of a trip I would carry 2 spare tires.
2. It probably wouldn't hurt to have a road side reflector of some shape form or fashion just in case you brake down at night on a narrow shoulder on a highway or road for that matter. It can be really dangerous while being parked on the side of the road.
3. Grease the bearings on the trailer if they haven't been done lately.

We had a problem yesterday with a wheel bearing but not on a boat trailer and the trailer wasn't mine. We happened to be following my uncle up to deliver a heavy load and the trailer wasn't up to par to say the least.
4. If your not up to date on your vehicle insurance and don't know what it will cover on towing check that also.
5. Make sure your safety chains on the boat trailer are securely mounted to the trailer.
6. If your boat trailer lights are not the LED style bring a couple of spare bulbs.
7. I would also check the boat to make sure it is sitting on the trailer properly, winch is working properly and locking properly (if you have a bow eye). Depending on how old the trailer is, I would carry a couple of extra
bolts or pins to place on the trailer hitch lever.
8. Make sure your transom saver is fastened to the motor, or out drive with something. I mainly use a strap although I have also been known to use a bungee cord.
When towing a trailer as the other guys mentioned already, I also put a couple of 2x6s in the back of the truck and carry a floor jack. I also carry the right size socket with a ratchet, just in case I want to check the ball on the back of the truck to make sure that it is tight. I'm probably going way overboard here but it wouldn't hurt to have spare lug nuts also. Allot to inspect and get prepared for before traveling a long distance. My longest travel was to North Carolina and back with a boxed in U-haul trailer and we had that thing loaded down

. Then my father was a truck driver so I guess I got some of my safety and knowledge from him.