[Torchwatch]

A good string suit, a swim suit in which the fabric is sewn onto an external string of elastic around the waist (which may be in a fabric sleeve), need not fall off in a rough sea so long as the elastic is of good quality and the correct size for the swimmer.
Any swimsuit without a waist cord may slip down or off when diving, but good firm waist elastic should keep it on and up the rest of the time.
Having the front of a suit lined or unlined will alter it's appearance but not alter it's ability to stay on, the leg elastics see to that. Never try to cut out the lining from a suit. you will almost always nick the fabric of the suit and permanently spoil it. Better to buy a suit made unlined if that is what you want.

A good string suit is harder to make but fun to wear, pity they are so rare.