My advice to you would be to use a combination of strokes for your 400 yrd swim. You can still get 3 points for your swim if you complete it in 11 mins. I would think that you are getting warn out so much because you are doing front crawl all the way. A fairly fast stroke but very tiring and if you are not perfect at it it can be a very inefficient stroke to use - as others are saying
Try doing breast stroke and back stroke whenever you get tired of front crawl. You will find that this makes the distance a lot easier to accomplish. Freestlye will produce a lot more lactic acid in your muscles caused from anerobic respiration - this is what makes your muscles hurt and tire quickly. Breast stroke and back stroke will help your body to respire more aerobically and will give the lactic acid a change to go away.
You can't sprint a 400m race like you can 100m race. Same thing with swimming.
I think that this method of taking it a little easier on yourself, not trying to score so hiigh ( I am sure your other scores will make up for it!) and changing strokes will see you through. Good luck!
I did my tests all together one after the other in choppy seas in a rain storm and swallowed a lot of water (and nearly gave up several times) but managed to scrape by and I am not that physically fit at all and I know my swimming technique is not so hot either but by alternating strokes and only using freestlye (front crawl) when I could manage it - got me through. I don't know what my score was but I passed. You are not trying to impress anyone remember!
You really don't have to be a great swimmer to pass these tests! Just don't push yourself too hard and avoid going too fast at the start and running out of steam towards the end. Ask your intstructor to tell you how many laps/ distance you have left and what your time is a few times throughout the swim so you know how to pace yourself.
Best of luck to you.
Remember - you are not allowed to wear a wetsuit or complete the rest of the exercise later (by PADI standards). If it is not completed it must be restarted and finished in full.

You can't sprint a 400m race like you can 100m race. Same thing with swimming.
I think that this method of taking it a little easier on yourself, not trying to score so hiigh ( I am sure your other scores will make up for it!) and changing strokes will see you through. Good luck!
I did my tests all together one after the other in choppy seas in a rain storm and swallowed a lot of water (and nearly gave up several times) but managed to scrape by and I am not that physically fit at all and I know my swimming technique is not so hot either but by alternating strokes and only using freestlye (front crawl) when I could manage it - got me through. I don't know what my score was but I passed. You are not trying to impress anyone remember!

You really don't have to be a great swimmer to pass these tests! Just don't push yourself too hard and avoid going too fast at the start and running out of steam towards the end. Ask your intstructor to tell you how many laps/ distance you have left and what your time is a few times throughout the swim so you know how to pace yourself.
Best of luck to you.
Remember - you are not allowed to wear a wetsuit or complete the rest of the exercise later (by PADI standards). If it is not completed it must be restarted and finished in full.