I think the most obvious answer to what to do for a stretch mark cover up is to, well, cover up. Just wear clothing to cover the spots you do not want seen. If it is summer time, though, you may not be able to do that out at the beach or depending on where the stretch marks are, at picnics or what have you. If it is hot you may want to wear as little as possible.
If you are planning on a beach outing though I do not think trying to use make up or something like that for your stretch mark cover up would work too well if you are going to be in the water for an extended period of time either. It would just wash off and if it isn't a water proof type of make up then you would get in the water and just stand there in a cloud of washed off make up. I would think that would be far more embarrassing than the stretch marks would be.
I have heard that there are waterproof body cosmetics that will work very nicely but again having to put make up on your belly or where ever your stretch marks are might by a little uncomfortable just in itself. If the make up is a heavy type like theater make up it won't let your skin breath and might just get all over everything. Again, more embarrassing than the stretch marks themselves.
Some say that a nice tan will help diminish stretch marks. So start going to your local tanning salon during the spring to have a nice tan before summer or use one of those self tanning lotions. A spray tan is almost instant so if you have procrastinated the least little bit then you could have a nice tan by tomorrow. Spray tans or self tanning lotions are safer than tanning under the bulbs they have in those tanning beds or lying out in the sun. Make sure you have an adequate sun screen if you choose this route.
If you do have 6-8 weeks before the swim suit season starts then go get a nice thick cocoa butter or vitamin E oil to rub into your skin two to three times a day, especially after showering when skin is already softer and more absorbent. Keeping the skin soft and pliant is the key to diminishing the stretch marks you already have and making sure you are not getting any more. Drinking lots of water will keep skin hydrated, too, so make sure you are getting enough every day.
Even the type of bathing suit you wear can be a good stretch mark cover up. There are many flattering styles that come in one and two pieces. How about a nice tankini? Or a bathing suit that has a pair of boy shorts instead of the traditional bikini bottom. Pair any one of these with a sarong or other stylish cover up and you could be the object of many looks of appreciation rather than looks of disgust.