$99 for a two tank dive is certainly a fair price. Do most cruise lines offer scuba as a shore excursion? My one and only cruise was with Windstar several years ago and we were able to do it from the ship.
The big-ship cruise lines will all offer scuba shore excursions in the appropriate ports.
Normally we avoid ships' excursions like the plague, but for scuba diving I think it's the smart move, for the reasons I mentioned in my earlier post.