Is Discovery Cove an all day park or morning till lunch sort of thing? I understand it is fairly slower paced than the other theme parks thinking about doing it as a breather in the middle of our trip.
We arrived before DC opened and we were probably one of the last families to leave that day. I don't know how many times I told my dh that I wished we could stay longer.
Seriously, don't try and rush through the day - take the whole day and enjoy every minute! Before we arrived I thought the same thing as you - is there really enough to do to keep everyone busy all day? I honestly don't know where the day went, it went by so fast - but it was by far the best day of our 2 week trip.
I thought we might be done early and had made 5:30 ADRs. I ended up cancelling them. Even though it is slower paced, you will fill up your day. I think you would be very disappointed if you only went a 1/2 day.