I wouldn't blame you one little bit for not doing it. That will be a cold, miserable race. Having run a marathon last month in freezing rain the whole way and temps in the 50's- I was miserable and I just couldn't warm up. I can't imagine swimming in that cold water, riding in that cold/pelting rain and then trying to convince the muscles to run.
If you don't do it- none of us will think any less of you. I say go with the indoor brick and save yourself a week of recovery. Plus- you have another race in 2 weeks anyway. Save it for that one!