To my knowledge gold isnt effcted by salt water...as like other medals and silver. Cobs of silver and copper are encased in marine growth. Gold however lays on the ocean floor the same as it did the day it landed there. If indeeis crusty growth its because it was on another medal like silver... and it somehow did transfer. One of the main reasons gold is so cherished it does not corrode.
Not sure if you overpaid but if so, it was probably very close. Gold has gone up recently. Seems to be a nice doubloon. I've never been able to get any Colonial era 8 escudos from any country (Vice Royalty) in any condition. According to Krause, it's .875 fine gold. Don't feel bad, I just learned that my Peru Arequipa 1839 4 reales is only .666 fine silver & not .903.