Performers who claim to reproduce historical situations as closely as possible must be measured by that ideal. If they restrict this claim to the acoustic result, they need not wear sailors' clothes or beards. However, if they use harmonies or other musical devices thta were unknown at the time they envisage (usually the time when the song was created), they may still make good music, but will fail their claim. The situation is similar to a picture restorer in an art museum who uses modern colours - a failure!

On the other hands, many shanties and other songs have never been sung in the situations they describe. In such cases, a really authentic performance may well include a piano, five-part harmony etc.; present-day audiences may prefer "fake authenticity" to "authentic fakes".

Have shanties ever been sung in the upper octave in genuine situations? I certainly think so, not only by those few women dressed as men, but by young boys, e.g. from orphanages. Some workhouses sent their boys to ships as a punishment, or sold them. Others were so bad that inmates deemed themselves happy to escape on a ship.