How exciting for you!!!
I carry my gear in a Tamrac Adventure Backpack (I think Adventure 7; the upper pouch in the 6 was too small for anything useful). For your purposes, the pouch may fit your rain gear, but not all of your family's. Also, although I LOVE my Tamrac backpack, backpacks are not good for "quick access" to your camera, if you want to be able to whip it out fast. It fits my dSLR with lens and up to 4 other lenses, hoods, batteries, memory, cleaning stick, etc. in the lower section, freeing the upper pouch for other stuff, like my "how to use my camera" book, or my lunch.
I get quicker access with my sharp looking Tamrac...um...looks like in the catalog they call it an Express pack; mine doesn't have side pouches like in the catelog, and can only hold my Canon dSLR (I have both a 40D and a Rebel XT) and whatever lens is on it, MAYBE an additional lens if I take out padding.
Here is the online Tamrac catelog:
http://www.tamrac.com/3548.htm
A GREAT place to ask for equipment advice is on the DIS photography board - check it out for great pictures too!!!
http://www.disboards.com/forumdisplay.php?f=97
Have a WONDERful cruise!!! Happy shooting!
