There are no specific certifications or training needed to become a steward/stewardess other than basic safety training that meets the STCW 2010 (Standards of Training, Certification, and Watch keeping) requirements. For details about the STCW 2010 training requirements, read the page about deckhand training.
Another recommended training option for stewards/stewardesses is silver service training. It is offered by most yacht training schools, and is a two- to three-day program. Students are taught the fine art of providing high quality service to passengers.