2.3. Why do my legs sink when I glide, and how can I improve it? Can I use a pull buoy or fins?

Do your legs also drop like two blocks of concrete every time you try to glide? You most likely have very muscular legs, and those muscles are pulling you down. It won’t be fun — it’ll feel frustrating and like “it just doesn’t work” — but that’s exactly why you need to include gliding exercises in your practice.
If it truly doesn’t work, then it definitely makes sense to use swim gear. Swimmers most commonly use fins or a pull buoy to improve body position. The pull buoy is a good option; fins are not as ideal — I’ll explain why in a moment.
At the beginning of learning front crawl, I recommend getting a pull buoy and using it for gliding exercises. However, the sooner you stop using it, the better — the goal is to learn how to manage body composition as it is, not rely on a floatation device between your legs.
Once you learn how to swim, I’d only use the pull buoy for specific training — either for body rotation and balance drills, or for arm strength training, meaning: pull buoy between the legs, arms pulling, and legs trailing behind without any kicking. Do not use it just because it feels easier or because breathing feels better with it. That only masks the fact that your body position needs work.
You can buy a larger or smaller pull buoy — the difference is in how much buoyancy it provides. You can place it high between the thighs or lower near the ankles — each option activates slightly different muscle groups. While gliding, try not to kick at all and keep your ankles close together. A common mistake I see is people lightly kicking with their crawl kick, which doesn’t engage or strengthen the deep muscles that are essential for proper gliding.
A basic drill for gliding is lying still on the surface, tightening your glutes, core, and the back of your legs, and trying to glide for at least 10 seconds while keeping your heels at the surface. If you can’t manage that without a pull buoy, place it between your thighs and try again. If you place the pull buoy lower, between your ankles, you can still do the same exercise, but it’s a bit less stable — and you need to make sure you don’t arch your back like a banana. If the pull buoy is between your ankles, then for the same basic gliding drill, tighten your glutes, core, and the back of your legs, and try to glide for at least 10 seconds while keeping your heels and hips at the surface.
Now about fins – be careful with them. Fins are a good servant but a bad master. They make things easier and forgive a lot — maybe you’ll swim crawl faster with them than without, because their extra speed keeps your body in a better position. But they won’t teach you how to swim. To put it bluntly: if you can swim a pool length with fins but not without them, you don’t know how to swim.
Use fins when you’re doing drills focused on breathing, on legs, or on rotation. But they’re completely unsuitable for improving body balance and position.
I had a swimmer named Ruda, who I inherited from another coach. Ruda wasn’t skilled at crawl — more of a strong guy, not really athletic, more of a bulldozer — and he wanted to try his first triathlon. He picked a short one, with just 400 m of swimming. And his previous coach had taught him mostly with fins, saying, “You’ll have a wetsuit in triathlon, so no worries — it’s basically like swimming with fins.” Well, it wasn’t.
Ruda brought his new girlfriend to that first triathlon, and she brought her parents — to meet him for the first time. And then came disaster. Ruda jumped into the water in a wetsuit, struggled for about 100 meters, until he panicked and had to be rescued by a lifeguard boat. Not exactly the first impression you’d want to make on your girlfriend’s parents.
That’s when I got him — broken and discouraged. We started drilling body position, including floating and balance work in every session, even though he really couldn’t do it at first. He suffered through it, kept sinking like a stone, wanted to quit a thousand times. But after a year of training, he finished his first Ironman: 3.8 km swim + 180 km bike + 42 km run. And he’s done several more since, all over the world. He made it. But he had to learn to swim without fins.