Dave,
If you can sorta swim, you'll do fine at the swim test. I don't remember the exact swim requirements, but they were not hard. If you cannot pass the swim, you will be pulled from your OCS class and put into remedial swim, which sucks, but they WILL teach you how to swim and you will graduate. I've never heard of anyone not finishing because of their inability to swim. If they can teach this one dude in my class who had 0% body fat and musta weighed 230# (all muscle) to swim, they can teach anyone. He couldn't float to save his life, but man was he strong, he beat the water into submission.
hope this helps. you might find someone on here who is a more recent graduate to give you further details on the swim test. You might be able to find it online too, OCS requires a class III swim test.