The time I was the terror in the deep.

I have a story that that I think will fit this subreddit nicely. It took place a few years ago.One summer I went with my girlfriend to stay with her family for the weekend. They live in a rural New England community, right next to a fairly large lake. Naturally, they spend a lot of time on the lake during the summer and that's mostly what we did over the weekend.One afternoon we went battle-tubing with some of her family's friends. For those who don't know what that is; battle-tubing is when you tow two tubes from a wakeboarding boat, and the people being towed try to knock each other off their tubes. It can be pretty difficult to stay on the tube without someone trying to knock you off as you're being dragged through the water at about 35 mph.Anyway, we had just turned around to collect the two tubers who had just fallen off in a particularly wide open part of the lake. We decided to take a break for a few minutes and just swim by the boat. The water was pretty deep there, not sure how deep but I couldn't see the bottom.Now it's important to mention that my girlfriend isn't afraid of deep water, but she is afraid of big fish. It's also important to know that I'm a jerk. I always try to sneak up on her underwater and grab her, but I never get away with it, she always sees me coming. But this water was particularly dark and deep.I remember realizing at one point that no one was paying attention to me and thinking "this is perfect". I quietly sank under the water and dove down until I could just barely see the surface. I've spent almost every summer day of my life in the water since I was little, so I'm comfortable with diving down and I can hold my breath for a pretty long time. I only had to go about 8 or 10 feet down, the water was pretty dark and murky. I also couldn't see the bottom, I'm still not sure just how deep that part of the lake is. I had gotten an idea of which direction she was in before diving as I knew I wouldn't be able to see well down there. I just swam in that direction until I could see feet kicking above me, and picked out feet I was pretty sure were hers. Once I was under her I swam up and grabbed her ankle. I still couldn't see that well even close by, but she just kicked away and I couldn't hear anything, so I figured I hadn't fooled her. Still, I decided to just wait down there for another 15 seconds, I thought maybe that would convince her it hadn't been me. When I eventually did come up everyone looked at me and started laughing, all except my girlfriend who looked pissed. Apparently she didn't know it was me and had screamed at the top of her lungs, I was just so deep under the water I couldn't hear it. However, I had stayed down so long she had realized I wasn't there and it must have been me, and she was afraid she had kicked me in the head, knocking me out. So I ended up giving everyone a scare until I came up, which I did feel bad about. But in the end it did make for a funny story.