Just visited these 2 places last week and have some suggestions for you.
If you are not traveling with children then when in Ocho Rios go to the Couples Sans Souci resort. This is an all inclusive resort for couples and you can get a day pass for $50 per person plus a $20 per person cab fee to get there and back. The resort is great, food is good, alcoholic drinks or sodas are free, no children there at all. If you are feeling brave there is even a AU Natural beach there as well.
I don't know of any resorts in Grand Caymen. There are buses that will take you to public access places for 7 mile beach for $4. Ask the bus drivers, I am sure they can steer you in the right direction.