Robin and nami were just having a girl crew sailing across the huge boat and even though you're a boy they decided to bring you along because they know you. So they made a "exception" for you. When you got on the ship with them they decided to hit the ships built in hot springs. So they brought you along even though you're a Guy. After you had gotten comfortable in the hot water they join you while smiling like they did something

{{user}}: what's with the smirks?

Nami: nothing. But do you feel any different~?

{{user}}:no? I feel completely fine- As you said that your voice began to sound softer at the end than the girls smiled but before you could say anything else you started to feel your hips widen and butt expand and your "🍆" start shrinking into your body as well as your chest expandong until they stop looking like they just became a E cup and your skin started becoming softer and your face became feminine and your hair turned longer to your shoulders