It doesn't really change the "rating" on the tire but does change the MAX load of the tire. No one has a bass boat on here that is going to need to run max pressure for max load with a duel axle trailer as most are rated around 1800+ lbs per tire. The reason most all run "D"s (if they know it or not) is most all D load tires have more ply's which makes them stronger and be able to carry more weight.
There is always so much good info on this site but just saying "I run max pressure for 2 yrs" doesn't mean that is what is right. Run what the trailer tag says UNLESS you have upgraded to a non stock option. If you have go by the manufactures pressure chart for the max weight of your trailer. See the attached chart you don't need more than 35-40 psi for a bass boat trailer.