Here's one for you...

Alright, this morning at swim practice I got my butt kicked!
First things first, if you are a swimmer and you want to get faster buy an ankle band (or just make your own...but it needs to be a thick band that is difficult to get on so that it holds your ankles together tightly while you swim). I have had one for a long time and everytime I do a pull set I use it...it makes you much stronger and forces you to use good form and your core muscles to swim. The band makes swimming MUCH harder even when you have a buoy and paddles.
Now, for the next challenge you need to swim with the band and paddles ONLY (no pull buoy). You will feel like you are sinking big time! The key to being able to make forward progress somewhat efficiently is to swim through your CORE, which means you have to use you core muscles (EVERY muscle in there) to stabilize yourself and propel yourself forward.
This workout isn't for someone just starting to swim or someone that hasn't done much pulling...
Warmup plenty with swimming, stroke, kicking
Swim 10 x 100 on 1:40 (band and paddles only)
Swim 3 x 200 on 2:50 (band, paddles, buoy)
Swim 20 x 50 on 1:10 (Swim hard without any equipment)...you will feel like you are flying...keep your focus on your core, good form, and a fast kick.
Cooldown
*Over the course of swimming 10 x 100 you should try to find an efficient way to swim. You will notice how little you use your core when you swim. It will feel like you can't get your core muscles to fire/activate and, truthfully, they aren't able to b/c your body isn't used to using them during a swim workout.

Give it a go and let me know how it goes!