They can if they are comfortable with it. I wear a one piece suit all the time and no one really cares or even notices.I agree with the above answer it is their choice if they choose however I need...
You can get one-piece swim suits that have nice patterns and designs on them.
However, the monokini seems to be in style right now. It is like a two piece swim suit except it is joined at the front....
The term "one piece" is what the series refers to as the treasure left by the previous Pirate King, Gol D. Roger. If you are referring to 'what is the treasure', then nobody knows yet. It is...