Yes, you should always log your crappy dives as soon as possible. In your case you knew what the problem was (Underweighted) Why you had this problem (Switched from Tropical to cooler water diving) And what you you needed to do to fix this problem. (Add X pounds, and being some extra just in case...) Not much this info will help you at present. Now into the future. If this same situtation where to happen again, you can refer to this dive and say, shoot I need to add X weight to this dive...Hopefully you will have listed the exact amount of lead you guessed you needed if not, hopefully you will do a weight check and then grab some extra weight from the car or buddy before the dive. Have fun....