How to tan at the sea without getting ta lines (topless is not an option)?

I have my matric farewell in six weeks and I don't want any tan lines showing with my dress on. I'm currently starting to tan naked. Now we are going to the sea in three weeks. I want to go swim in the sea ad enjoy myself but I don't want bikini tan lines. Even if I use sunscreen whit my bikini, there will be tan lines. Any ideas??