Try the Freya Pier suit. I have it in what was supposed to be hot pink but really looks cherry chapstick red. The tie is removable and trust me, if it can hold up my sisters, it can support yours.
If you simply must have a halter top, go with one that has structure, preferably one that's built similar to a bra including adjustable hooks and eyes in the back.
The website where I bought them is no more. Or at least it was in the stages of being no more when I last looked.
If you google the names of the suit, you should be able to find them. I know they aren't the cheapest things but I swear they last and they are worth the price.