We did the swimming with stingrays excursion in Antigua when we went with Ocean Village (part of the same cruise group) and it was excellent. You are taken out by boat (at some speed) to a pontoon where the stingrays are kept in a fenced off area. The water is about 4ft deep so you can walk around. The local staff took great care of a very nervous non swimming lady who enjoyed it so much she was last out of the water! With regard to local tours places such as Grenada have mini buses that duplicate the ships tours for less but a lot of places just have taxis who will do a similar tour but I'm not sure about the prices. Don't forget that if a ship organised tour is delayed the ship will wait for you, but if you have done your own thing and you are late it will sail without you! Have a nice holida.
Thanks for that bradley stringrays already booked anymore tours you can reccomend.