Depending upon which error the manufacturer made, blisters don't always show up immediately. Sometimes they just show up unexpectedly. If the layers of fiberglass mat were dry in some areas, when the boat pounds across the water, and because the gelcoat was not bonded well, or at all with the fiberglass, the gelcoat may become weakened and it will flex. This can in turn cause a tiny little stress crack to appear on that area of gelcoat. The small stress crack then allows water to be forced in and under the surface of the gel, which in turn causes a blister to appear.
Pressure washing can have the same effect.
But again, it is not the fault of the marina or the guy who does the pressure wash for you. If the foundational structure and the surface of the hull were what they should be, then pressure washing will cause no problems for your boat. If the fiberglass had too much hardener put in it during the manufacturing process, when the boat then pounds across the water, because the fiberglass strength was compromised, it can shatter because it was brittle. When the fiberglass shatters underneath, the gelcoat has no choice but to crack or to blister. There are several methods to take care of the blistering problem.