Ahhh another case of "morning after regret"!
I've done that more than once. The voice in the back of your head was screaming, but you HAD to have that boat!!
I found out (on the internet-so it has to be true) all the good poop on trailer tires.
You take the weight of the boat and trailer, guess the weight of the "stuff" inside- fuel trolling motor, coolers etc..
Take the load rating of the tires
divide the total weight by the # of tires and Bob's your uncle-there you go! If it's under your tire rating off you go, if not, well...........................
Make sure any old tire is inflated right AND not dry rotten. (that's the case in most tires, lots of thread,but the cracks)!
When the tires are "iffy" I'll go some, check the heat of the tires, go some more. If they all the same temp coll, is the center is hot lower PSI, outside add PSI.
Joe
Always remember any advice I give should be taken with a can of beer er 2!