Ok, I've made my decison, going to GC 1st week in April and staying at the Marriott. Please suggest a few good restuarants where I will be comfortable solo at night, and also a casual place for lunch and drinks, hopefully on the beach. Also, would like to take one of the sunset cruises but afraid it will be all couples????

We took the Jolly Roger sunset cruise and it wasn't all couples. Families and yes, couples, but I wouldn't have felt out of place if I was by myself. The crew was very cool too, so you could chat with them. Would recommend just the appetizers though, dinner didn't look worth the money.
http://www.jollyrogercayman.com/

Try Coconut Joe's...fun, casual place with good food and drinks.

Also go to the Reef Grill/Royal Palms (on the beach)

One last thing..Coconut Joe's is right across from the Marriott....

I would sign up for a diving or snorkeling trip the first tday that you are there. THe divers are all very friendly and offer to take people out to a lot of the local places if you want to. There are so many great rest. there and the good news is that the hotel has a list of all of them with menus. When I stayed there, I ate lunch at the hotel..so cute...and ate dinner out.