When my wife had her 100th dive, I knew it was going to be while we were diving day boats off of Kona, HI. I made her a commemorative shirt in advance. I also alerted the boat crew (Kona Honu Divers) and they created a "certificate" on the computer and had everyone sign it. After the dive we had a little ceremony on the boat, presenting the shirt and certificate. The other 14 divers aboard cheered and got into the whole thing. It was great fun. On the other hand, my 100th dive occurred during a week of training dives in local Aurora Reservoir and passed without much fanfare. 100 dives is a huge accomplishment. You should make a big deal about it!
DivemasterDennis