It bit down on her lower left leg, and foot.
With the help of her boyfriend, Rick Hughes, also 18, Foster made it to shore.
It was the first shark attack in the area in 20 years.
Foster needed 29 stitches but told "Early Show" co-anchor Erica Hill she's "doing fine," and that doctors say she didn't sustain any permanent damage.
Foster says she knew right away what was going on. "I didn't even consider anything else could have hurt that bad." Foster told Hill. "I could kind of feel all the teeth inside my skin, so I knew."
And, she was "nothing short of freaking out. I was so scared. I thought it was gonna smell the blood from where it had already bit me and come back. So, I'm just seaming at my boyfriend, yelling, 'It's coming back, it's coming back. I've been bit.' He didn't even know it was a shark. I couldn't even get that out."
Hughes says he "just heard her screaming and it just scared me to death. But I turned around and just grabbed her as fast as I could and tried to just get her to shore."
Was he concerned about another attack?
"I had no idea that it was a shark until we got closer to shore," Hughes responded, "but I didn't really think about it. I just wanted to get her out. … It was just crazy. I don't even know how else to explain it."
Foster says the water was so murky, "You could barely see at all. I could see the shape of (the shark) but I couldn't tell what kind of shark or anything."
Once on shore, Foster says, "It was about 30 minutes before I got to the E-R and got any medicine in me. They spent time just bandaging it and making sure that I didn't hit an artery or anything. So, I spent a lot of time on the beach and in the ambulance."
This wasn't Fosters first run of bad aquatic luck. When she was 9, she was bitten by a Portuguese man-of-war, which left scars on one of her arms for three years.
So now, says Foster, "I don't want to get back in at all. I know the odds, you know, are in my favor that I probably won't get bit again, but I obviously have pretty bad luck, so I'm sticking to pools."