I did one 1500m swim in rough 52 degree water. I had no neoprene hood or booties but sure wish I did. There were a slew of DNFs, and it took me 15 minutes before I felt my feet again. The air temp of that race was in the 70's-80's, so I had the fortune of quickly warming up.

If the race directors allow booties, you might want to try to secure a pair. 400m shouldn't be too bad, but running to transition on dead tingly feet isn't fun.

If you are a decent swimmer, you should be generating enough body heat to handle the temp. If you have not yet swam in really cold water, try to go at least once before the race. Your body reacts in ways that you have to compensate for. You really have to focus on proper breathing as your lungs will naturally want to tense up. Focus on just doing your thing. Force yourself to maintain a regular stroke pattern. Fight any urges to breast stroke or stop as you will lose body heat.

For 400m you should be able to just go at it like a madman (or woman) and you'll completely forget the water temp.

I'd definitely go with the hood. The whole race is pretty short, I honestly wouldn't worry about arm warmers and such, you're going to be going hard the whole way.

I see the time value of armwarmers under the wetsuit, but getting them wet seems to make them just cold and almost not worth wearing for the bike... Rich, did you find they dried fairly quickly? Did it make the bike even colder for you?

I just did the CA half this past weekend like a few others and for a short swim like SF, you should okay with no booties or gloves. I had just a 3mm surf wetsuit and 2 normal swim caps, I survived. 2 swim caps would be a good idea. If you can get a neoprene cap, that might be good. If it is a beach start, expect it to be hard to breath the first few strokes because of the cold water. It will feel like your heart rate is at 250 because you can't get a good breath, but it will go away after just a few strokes. Just stay calm as you will be fine in a few seconds.