Most of what I would say has already been said about body position, frontal drag, free speed off the walls, etc. However, how do you get better at streamlining? There are probably a ton of drills out there, but here are some of my personal favorites:
1.
Going the Distance: Push off the wall and just glide as far as you can. Work on your arms, legs (including your butt), and core being really tight. Focus on reaching towards the other end of the pool with your hands and arms. The tighter you are, the better.
2. Streamline Bobs:
Here is a video, but I like doing them in the deep end to increase the difficulty level. If your body position is good, you will go straight up and straight down. Really fun drill.
3. Diving into the pool: Seems weird, but if you can jump into the pool from the blocks this is ideal. If you have a good streamline and good body position, you can go very far underwater after jumping in. Just make sure the pool is deep enough...no backboards if at all possible. If you can practice starts safely, that would be great. Gets your body position right and helps you have a better feel for the water.
With that said, I don't think streamlines will help you a great deal with open water swimming. It WILL help your body position which WILL help your open water times. I sometimes shorten up my streamlines off each wall to mimic the open water timing. It's a trick I learned from a guy who did the English Channel swim.