Totally novice swimmer...need advice

I would suggest going slow for the duration of the pregnancy.   Work on form for now.  When you go into oxygen debt, so does your baby.  After the baby is born hit the training as hard as you want. 

Also, 750 yards for a beginner is quite good.  And you are right, biking and running fitness do not correspond to swimming fitness.  It's the nature of the beast.

If you are out of breath, you are probably not  breathing properly.  Make sure you are exhaling under water, so when you turn to breathe, you are not exhaling and then inhaling.  No time for both.  Also, a common beginning swimmer mistake is not putting your head in the water, and flipping it from side to side.  You mentioned that you were taking lessons, so I hope your instructors are not letting this happen.

If you are getting ready for tri swimming, the key is to glide with each stroke.  Don't worry about a fast stroke.

As everyone has said - keep focusing on technique and learning to breathe properly. Once you get that straight you'll find that you can quickly advance. You'll find that once your form is relatively good, things just click into place. I went from never doing more than a 100 yrds at a time (and struggling to complete a straight 100), to completing a straight 500 yrds one week and then going on to a straight 1000 yrds the next week (with plenty of gas to go further).

Take some lessons, find some online resources (swim smooth, TI, BT all gave me some good pointers), and just keep at it.

It may be difficult to distinguish, at this point, how much of your breathlessness is due to inefficiency and how much to pregnancy. But it's fundamentally unimportant which it is. Keeping your HR low and avoiding excess breathlessness, etc. - particularly in an activity for which you don't have prior adaptation - is likely something your OB would support(?)

Swimming definitely is a good activity to do at the moment, yet it's probably not an ideal time to be learning. This is because balance is the absolutely essential foundation of swimming efficiently. And the amount of weight and mass you have in your belly at the moment is, let's say, "inconvenient" to learning balance. Your balance characteristics will change greatly after you give birth. At that time you can develop skill and self-awareness that will be of more enduring value.

My best non-professional (i.e. on the medical side) AND professional (as a swim coach) advice is to swim the distance and effort level that allows you to complete a swim segment with a level of comfort where you can go again after perhaps 5 "cleansing" breaths.

Don't time these efforts. Focus entirely on aspects of the stroke and whether they increase your comfort and ease. Start with these three Stroke Thoughts:
1) Release your head until it feels weightless. Neither hold it up, nor push it down. Just let it go. Is it now neutral? Is your head-spine line straight? Are you looking down, not forward? Good.
2) Rather than pushing water back, use your hands to "lengthen your bodyline." (One will be pushing back while the other extends forward. You'll simply pay attention to the latter, not the former.) Try to do that with a degree of leisure. Try to make your extending hand/arm feel weightless. Watch for bubbles and try to minimize them.
3) Listen to your stroke. Do what you can to make it quieter.

Focus on only ONE of these at a time. Give it your full attention for 10 minutes or more. It will take at least that long to get used to such targeted attention and to make improvements. Compare your perception of effort and how long it takes you to recover after a length with how you felt before focusing on this. Then move on to another one.

When you can easily swim 4 or more repeat 25s with a 5-breath recovery between, then try a 50.

For a visual aid to the points I describe above check out the video in the link below my signature.

Enjoy.

I am the opposite of you - I have been swimming since I was 8 months old and am part fish.  BUT, I am a totally novice runner.  I will tell you what I recently learned about running and it might help you in the pool - I was reading on here some advice that I should keep my heart rate much lower than I was in order to properly train myself up.  So I SLOOOWWWED down, to an almost excruciating pace, and suddenly, rather than feeling like I was going to throw up after 4 minutes of running, yesterday I ran for an HOUR - straight!!!  No walking!!  I kept my heart rate below 150.

As a generally fit person, I am guessing that you are hitting the pool with the same gusto your pre-pregnancy body could handle running, and that's why you are so out of breath, just like I am with running.  Try slowing it down - you should be able to find your groove just as you do with running, but it will seem painfully slow until you get more used to it.

Also, you're 7 months pregnant!! Cut yourself some slack!!  When I was pregnant, I would get out of breath walking too fast! LOL.  And as someone else mentioned, if you get oxygen starved, so is the baby, so double reason not to do that!

Good luck!

Wow..thanks so much to everyone for all the advice and encouragement.  After reading your responses I headed to the pool this morning.  I slowed WAY down and did not worry at all about time.  I focused on staying relaxed, comfortable, and trying to glide.  It went much better.  My husband (also a runner and novice swimmer) and I bought the Total Immersion book this afternoon and are fighting over it--so I will start reading it soon and sort of "start over" using the concepts and drills in the book...knowing that I will need to revisit everything once the baby comes.