Chryo treatment is something that needs to be done by someone who has the right equipment and knowledge of the particular material that's being treated.
We had some props chryo treated by a local guy and the props lost all the hardening and they bent like they were made of aluminum.
Chryo treatment is frequently part of a hardening process. A typical hardening process may entail annealing, chryo, and precipitation hardening. There are very specific guidelines to these hardening treatments which have strict temperature control and time parameters.
I have learned some of these guys have a bottle of liquid nitrogen in the basement and will tell you it's the answer to all metal problems.
Whoever you work with, make sure they are familiar with PH (precipitate hardening) stainless steels ie; 15-5, 17-4, 13-8, etc. They should be able to give you information regarding the advantages of their treatment.
Kennorth, your boat has either the 440 or 480 Yanmars, right? With those motors and the drives you have prop breakage should not be an issue. You need to check the props from time to time for nicks. If you find some they needs to be addressed, especially on a surface drive.