3)Abundance of Fish: If you mean for snorkeling, yes there are a similar amount of fish compared to the VI's (STJ) more than you would find in the BVIs. Overall comparable snorkeling to STJ (especially on the North Side where Rum Point is), better than the BVIs (including Anegada and VG). Check out/Google "Testudo's Snorkel Guides" or the sister site Cayman Snorkel Guides for info on where to go. If you meant for eating, same deal. Mahi mahi, Wahoo, Grouper, Smapper and Tuna are the local eating fish.
4) Costs: Like many islands everything is imported. There are no sales or income taxes in Cayman so the majority of revenue is raised solely via duty. Unlike the USVI or BVI, Booze is especially hit hard with and over 40% duty on average <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" />! It pays to purchase some favorites at the duty free store at your departure airport of last connection airport. Unlike the USVI, purchasing duty free liquor is only availble for outbound persons, no stocking up like at the STT airport or K-Mart<img src="http://www.traveltalkonline.com/forums/images/graemlins/duh.gif" alt="" />. The CI$/KYD$ also exchanges at a 25% premium to the USD ($1USD=$.80KYD or $1KYD=$1.25USD). Grocery items are comparable to the VI's and Manhattan <img src="http://www.traveltalkonline.com/forums/images/graemlins/Grin.gif" alt="" /> and dinning out is a bit more expensive, but not too much more than the "better" restaurants in the VI's.
5) Costs part Deux: There are plenty of places to enjoy drinks and meals that are not completely outrageous <img src="http://www.traveltalkonline.com/forums/images/graemlins/dine.gif" alt="" />. Refer to Cayman Good Taste menu guide for an idea of what is available and the costs (remember to add 25% to listed CI costs). Most fru-fru drinks on the island will be $10US or less and beers and cocktails $5-$8. If you drink beer, buy a case or more of the local brewed beer from the Cayman Islands Brewery on the way to Rum Point. A case is $35 US vs $45+ at liquor stores. Kaibo, Driftwood and Over the Edge are the local bars. Rum Point Club's Wreck bar is a bit more geared to touri$ts, but does have great Mudslides.

Hopes that helps.