I'm a very keen swimmer and these are my recommendations for swimwear and bikini brands that are really good quality and wear well:
Always on trend and made to a very high standard, French Connection's swimwear isn't cheap but it will last a good handful of summers in the sea.
Marks & Spencers
For the slightly older market, M&S clothes are almost as well made as its food. Choose them for classic looks in stately colours - they're also pretty good at flattering women's bodies.
The choice o the yummy mummy, Boden delivers on retro cool, vibrant colours and swimmer-friendly shapes.f
For the pro, a Speedo costume is hard to beat. Not the prettiest costume but certainly the most practical.
There's also a similar question on Blurtit here if you're looking for swimwear that is more affordable but still looks great: