Everyone is on top of the trailer but lets talk about the truck pulling the boat.
Don't use Overdrive unless you are on flat ground, going up hills put it in what ever gear you have under OD, this will depend on your tow vehicle.
If you tow through a lot of hills, take it out of OD before you go down the hills, keep it in a gear low enough your not getting pushed down the hill by the boat and not riding your brakes the whole way down the hill.
Make sure your oil, trans, coolant are in great shape before towing.
Make sure if your truck has over 80K you have replaced the serpentine belt, and the coolant hoses are in good shape.
Have the brakes looked at by a shop that does free brake checkes.
If you have Drums on the rear get them adjusted before the trip
Make sure you have a Good spare on your truck for the truck with the right stuff to change it
If you plan to tow at night make sure all the lights are working on the truck and trailer
Bring a decent flashlight
All this has to tow with how fare you have to travel but personally this is the short version of my check list before i tow long distance, such as this summer in Sept i will be towing the boat over 3K miles to and from AZ and honestly its not the trailer and the tires i worry about, its my Rig and how hard she has to work to pull that big ass Bestia up and down the mountains lol.