Here's what I posted in another thread:
I have the TYR Aquatic Resistance Belt and use it to swim in a 20 x 40 foot backyard pool. It's great for the most part. It has a piece of latex tubing that has a nylon strap loop at one end and a carabiner at the other end. The carabiner attaches to a nylon belt covered by neoprene with a buckle for adjusting tightness. The D-ring that the carabiner is supposed to attach to broke while I was swimming (I'm not that powerful and wasn't swimming that hard) but the nylon loop on the belt that the D-ring was held on with works just fine for clipping the carabiner to. I swam growing up, so maybe I'm just used to being bored staring at the bottom of a pool, but I don't find it that tedious or boring. I thought I might need to get an underwater MP3 player/earphones, but never got around to it and don't mind it. It's easier to motivate myself swimming with the belt than on a treadmill.
I swim a few times a week and rather than using distance to mark the workout segments I either use stroke/breath count or time. I know that 35 breaths breathing every other stroke is about equivalent to 100 yds. for me, so I basically do the same kinds of sets as in a pool, just using that stroke count as a substitute.
Measurement is the biggest challenge, both of how much (distance) and how hard (speed) you're swimming since you're staying pretty much in the same place. Because it's elastic, you can use your position in the pool as a way of thinking about how hard you're working. As you advance a foot or two farther away from where it's anchored, you have to work progressively harder to maintain that position. It's inexact, but you probably have a feel for what it should feel like at different intensities.
It does have one big benefit. Since you're stationary, you don't have the water flowing under your hips and legs as your move through the water to help keep them up. I was never a big kicker, but it wasn't really much of a problem for me either, since my body position was fine. But swimming with the belt, at first it required concentrated effort to keep my hips and legs up in the water. This made it natural to swim with a lower head (like Gary Hall Sr. has talked about on the forum here and elsewhere) and to kick harder. Now, when I swim without the belt I have a stronger kick and even better head and hip/leg positioning than before.
It's definitely better than not swimming at all, is a good addition to a normal swimming routine, and is a decent replacement when swimming in a real pool isn't an option.