My 10 yr old niece is in synchro. As a former synchro swimmer I help out from time to time.
Her club has two boys in it. They are new to sport but were welcomed by the girls.
Buoyancy issues are different for boys. Floating on their back takes more time and flexibility can be issue. Ballet leg moves can take time for the boys to master.
It is however a great sport for the boys to work on endurance and water skills.