I need some advice on training for a 400m swim on April 17th. The biggest issue that I have is, I do not have access to an indoor pool, and I am concerned with training outdoors. What is the best way to go about training for the swim in this situation?

What is the big deal about swimming outdoors? I do it four times a week and have no problem. Do what you have to do to make it through.

Just be careful. Be aware of currents, and have someone watch you if possible.

is your 400m swim outdoors? part of a sprint tri? if it is i would work on your concern about training outdoors, watch videos, read books, talk to others or on forumns to get over any anxiety about it...you are not alone. I wish i lived in TX so i would be able to get into a lake down there.......have you checked out all your local ymcas, gyms? some will give you a daily pass or month to month membership...good luck

What I am concerned about training outdoors is the water and air temps. The river I trained for my first tri in, the temps are low 60s and should be falling in to the upper 50s soon. I would only be able to swim there on the weekends and the occasional evening when I get off work early. Would getting a wetsuit be suitable enough to train with in those temps? What else do I need to be concerned with training in the winter time?

As far as gyms, the nearest one is a 40 minute drive one way and their hours are basically my work hours. I got a 10 visit pass last year and I have not used any of the visits yet. There are not a lot of options for training around where I live. I just want to be in better shape than last years tri. It was a 800m swim and it took me 32 minutes to complete it.

Have you considered any of the training aides that you see in the back of the tri magazines? I have no idea if they work and fortunately I have a pool within a mile of my house so I don't need to try them. Seems like your situation calls for some out of the box thinking. I suspect you wouldn't swim in that river too many times when it's high 50's to low 60's.

Consider finding some indoor water available on Sat/Sun and take a travel weekend now and then.
Find a destination with additional things to do to fill the other time you're not in the pool.
Travel to a weekend swim clinic geared for triathletes.
Over the long term, you may want to look at an endless pool, or even more drastic solutions (ie: relocate, change job). Or change your objectives to be more realistic if you won't be able to get in minimal training.
32 minutes is a very long time for an 800m swim unless it was against currents.
I would not want to swim unsupervised especially in harsh conditions (dark+cold, lake or river).