Fisrt, I assume you are a certified diver. If you are not, do not do this dive excursion. If you are certified , take heart in knowing that you will not really notice any difference between 45 feet and 90 feet, other than your air will be used up half again as fast as at the lower depth, so monitor your air supply. Also, have an awareness of depth and stay at or above the planned maximum depth for the dive. Younwill also be in a possible "narc zone" so if you feel any syptoms of nitrogen narcosis, ascend a few meteres and stay at that depth. Maintain neutral buoyancy. Finally, be sure to do a safety stop for at least three minutes at 6 meters or 15 to 20 feet. Prior to the dive check out the equipment provided to make sure it fits, works, and you are comfortable using it. Soon I hope you will go on a dedicated dive vaction to a dive resort , where you will have a much better time than on these cruise ship excursions, which can be a little rushed, seem to always hit the same often over-crowded sites, and are conducted in pretty large groups. Comfort and competence comes with experience and with familiarity with equipment. As long as you are an infrequent and casual diver, and using rental equipment, you really should limit your dives to those that present no unusual challenges, including new depths.
DivemasterDennis