You are going to have to use some common sense along with some testing of your own with this as the tyre pressure will vary depending on what trailer you have with which tyres and what load you are carrying.
The walls of the tyres should flex slightly in operation in order to give the boat/trailer some help with suspension.
Inflate until the tyre looks right (looking much like your car tyre does when correctly inflated) and what you are looking for is for the tyre wall to be as soft as possible to give the boat the best ride without heating up through flexing too much. So after a few miles preferably on the motorway(at speed)? check the temperature of the tyre wall by hand, if hot or warm they need more air.
If cold then they are not too soft - if the boat's not bouncing and having a hard ride then that pressure is suitable.
With the above in mind a good starting point for most of the single dinghy trailers we supply 36psi/2.5 bar would be a fair place to start. Heavier dinghies and Multi-stacking trailers may require more pressure, 8 inch wheels need a higher pressure than 10 inch.
As an example I have a Laser combi trailer with 8inch wheels which I run at just under/around 25psi.
Check the information printed on the tyre wall and do not exceed maximum pressure recommended