Just be natural, its seaside, its NOT swimming pool. so she can wear a t-shirt or topless.
my boys 3 and 5 do that too whenever they are at the seaside. To me, buying swim wear for kids younger than 6years old is a waste of money because 1stly, they grow up so fast, 2ndly its expensive for the small piece of clothing.
BUT unless you have enrolled her for a swimming class, then its a different story, only then I think swim wear is a MUST!