My legs sink also. If I swam only a 3:00/100 (half my regular speed) my legs would be nowhere near the surface. With a combination of a strong core, a an adequate kick and relative speed I can keep my legs near the surface. Like another poster said, swim some faster 25's. With a light kick you should be able to keep your legs up. Then progress from there.

quote bretzky]
Any tips how to achieve this? Or just work on engaging the core, glutes etc to become comfortable being flat in the water. If I put a band on my legs sink to the bottom.[/quote]

Did you look at the video I posted above?

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"Go yell at an M&M"

All of the talk to kicking makes it sound like it's the legs kicking that somehow keeps them up, same with the talk of going faster.. that's not the case.

The kicking is a set of drills, either with or without kickboards, that get you to stretch out and engage your core and become taut and that lifts your legs. Same with going fast - when you try and go really fast you naturally tighten up a bit and go more taut, which lifts your legs.

I had struggled with this for ages. I even did a 45 minute 70.3 swim (in a lake with a wetsuit) last year because i was dragging my legs. Have a look at the Tower26 stuff. it's been good for me. He's got a podcast - i think it's episode 4,5,6 that discuss these main elements.

When I learned this progression, we did it like this.

-Start in the shallow end, and face the other end of the pool.
-Put your face in the water, and arms stretched out in front of you super man style, hands inside your shoulders, palms facing the bottom of the pool
-Bring your feet to the surface, and try to float
-Engage your core by "rolling" your pelvis forward slightly, to straighten your back. (I have no idea how to explain this otherwise, google is your friend here).
-Keep your head in a neutral position, looking towards the bottom of the pool and in line with your back
-If you are having trouble keeping your legs up, use your core/ head to correct this. "Press" ever so lightly with your chest and head, play with your core (again i know I'm not explaining very well, but bear with me)
-At this point you'll likely want to breathe, ever so gently lift your head the absolute bare minimum to get a breath in, and place your head back down.
-The goal is to find an equilibrium where you are using your core to float on the surface indefinitely.
-Try to stay as relaxed as you can, tensing up will cause you to need to breathe more, and make it harder to float.
-Once you can hold that float for at least 3-5 breaths add a very gentle kick, keeping the same body position in mind. Keep that gentle kick until you can comfortably do 3-5 lengths without compromising position (start each length from a float, no pushing off)
-Working in increments, slowly add more kick, in 3-4 increments until you are at a good propulsive kick (for you).

This will take some time, but helped me tremendously. The video below also is a decent summary. This is also a great drill to do in your warm local Y pool, since you won't be really doing a lot of work.

https://www.youtube.com/watch?v=voU8kW68MME

A faster arm turn-over will reduce the stop-and-go action of a slower pace. You maintain momentum by adding, say, 5 strokes per minute. I sense my legs staying higher when cadence is higher but it obviously comes at the cost of endurance. I really think the solution is swimming with a tight core and strong hamstrings. My first 50 is typically 10 seconds faster with a taut body and a 2-beat kick. Like the faster turnover, I have trouble maintaining that 'tautness' and I start slowing down. Core strength is key.