It will be easy to see the really big ones, but the smaller ones will be harder to identify in the water
. The big blisters
will frequently weep when the hull
is out of the water and drying out.
If I was concerned about possible blisters, I would haul out
the yacht and have a look before I purchased the yacht. If I was concerned about blisters on my own vessel, I would just wait until the next haulout to get a definitive answer to the blister question.