I can understand your mother's reluctance as a sailor. My advice book an early morning passage (first ferry departs around 7.30 am) - if possible on the Kilimanjaro II or III - these were to two largest and newest ferries operating the route a few years back. The fares were around $40 US but have probably gone up since then. The flight over is about 15 minutes and if you get a window seat the views are impressive - especially if the sun is shining (ie no or little cloud). Comparison between the ferry and flying - it will cost an additional 30,-40,000 TSH to get a taxi to the airport and take around 25-30 minutes (of course if you arrive at DAR International airport you can simply walk to the check it). Arrival at Zanzibar - the ferry arrives about 1-1.5 km north of the old town so you can walk or get a taxi to your accommodation (note not all hotels are directly accessible by road); from the airport - its a 20 minute drive and the taxi should be about $10-15 US (or 30,000 TSh) - its negotiable. Good luck and hopefully you will enjoy my island paradise. Cheers.