If a yacht has air conditioning it will be noted in the description of the yacht on the yacht card. Only a few of the biggest monohulls and some of the catamarans will have this feature.
Please note, air conditioning on a yacht is not of the same standard you may be used to on land! AC on board is designed to be used when docked in the evenings and whilst sleeping and is sometimes not available in every cabin. The cooling ability is not as powerful as AC found on land and when used continuously during the day the chance of malfunction increases. During the day, jump up on top deck and enjoy the sea breeze!