When I went on holiday last i was 20 wks pregnant with my 5th child and prob a size 18 and in the day I lived in one of those skirt dress things that are dead cheap and in most 'general' shops (bhs, m&s etc) which is a long maxi skirt with elasticated drawstring waist, or you can hoik it up so it is worn as a strapless dress - covers a multitude of sins and is below the knee in length. They are normally sold in the holiday section, and might be described as a beach cover dress or something like that. Hope that helps!
And for the eve, I agree that maxi skirt or dress is the way forward. Plus I had some white wide length maternity linen trousers which I wore with a long line top and wedges and I felt quite glam in that and got lots of compliments. The main thing I think is to look happy and confident when you go out, and don't shy away from colour as its nice to wear brights in the sun!
Also Aug is ages away and I reckon you could well be in the 14s by then anyway!! Oh and most high street shops like m&s, john lewis, Debenhams etc will sell tummy control swimming costumes and 'lose 10lbs in 10 secs' ones, but they normally cost a fortune......