The recommended air pressure vairies with make, model and manufactor, but most are surprisingly low. For example my AIRE Couger tops off at 2.5 pounds per square inch, some floors have a PRV valve rated at 1.5 psi.
I purchased a special low pressure air gauge along with my first boat. This was very helpful in the begining. After a while I could judge the pressure pretty accurately by pressing a knuckle into the inflated boat.
Keep in mind that air pressure is a functin of temperature, the air pressure in the boat changes a lot when it's moved from the hot beach to the cold water or visa-versa. Be careful of boats parked on hot sand I have seen them rupture when warmed up in the sun!