I tow my boat all over the place, all the time and have learned a lot through trial and error.
The pressure totally depends on the load. Look at the max pressure and max load rating on the tires, then do the math to see how they should be set for your typical load, and add 10 to 20% for longer tire life.
So if the tires are rated to carry 2000 lbs (1000 lbs per tire) at 40 psi and you are carrying 1500 lbs: 1500/2000*40*1.1= 33 psi or 1500/2000*40*1.2= 36 psi. Thus 33 to 36 psi will be good for the tires and load in my hypothetical example.
If you are at maximum load for the tires (really common with "factory" boat trailers that are built to a price-point and often barely adequate when the boat is empty) I recommend running them at 20% more than the max pressure printed on the sidewall. It sounds like it would be bad for the tires, but it's actually easier on them because they will flex less and run cooler. Have a spare or two and plan to replace them every 5 years or less if this is your situation and you tow often.
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