Swimming in pea soup

Last Saturday, I did my first open-water swim. Remember, I've been swimming for only seven months. Prior to November, I was a lifelong closeted non-swimmer.
The lake swim was intimidating. Distances are difficult to measure and perceive. Swimming in the lake was a little like swimming in pea soup. Little fishes nipped at my ankle bracelet. ::Shivers::
I did a few drills parallel to the shore just to warm up. A few times, I stopped, realized I couldn't touch bottom and panicked only slightly.
This treading water thing eludes me. I work my ass off to keep my head above water, even when it's not that deep. Friends tell me it's my comfort level, but I don't get it. I'm fine floating on my back or stroking through the waves. My mom tells me it's because I don't have enough body fat. I'm gonna go with her answer.
Anyway, my friend, Keith, told me to swim out 16 strokes. I did, no problem. Flipped on my back, counted to five, flipped back over and then flailed my way back to shore. My heart raced as my arms stroked like a windmill back to shore. Trying to keep him in sight was a challenge, because in the pool, we're taught to keep our chins tucked, heads down. Now we have to lift a little, to avoid drifting. Keith told me to slow down next time.
Using the tempo trainer set at a slow stroke - 1.4 seconds - Keith sent me back out. Much calmer on the return. The swim out tends to be calmer. The swim back to shore becomes urgent because I know I'm returning home and my feet will be on sand again.
Last night, at the pool, I swam a 400 set at 1.4 just to see how it feels. I did it with no problem, and could have kept going for another 400.
As I swam last night, I thought to myself that I couldn't believe I had such a hard time with a 400 in April at the Tri for the Cure.
This Saturday, my friends want me to try a 600 in the lake. Think I can get one of them to paddle alongside me on a raft?
If I can keep the pace at 1.4, I'll make it through the 700 in July at the Flagstaff tri, no problem. I look forward to the day I swim a mile in the ocean and look back at this struggle and think, "I can't believe I whined about that."