Yes, it is true that the mini-suite balconies on Dolphin Deck are open and can be seen from above; but I have booked these cabins many times and I can tell you from experience that the passengers above have always been very respectful of the privacy of those below. Those above have to go to their railing and look down to see into your balcony; and this just does not happen on a regular basis.
However, if this is a concern to you, the mini-suites on Emerald Deck (one deck below Dolphin) are exactly the same, but their balconies are covered by the balconies on Dolphin deck; and they are less expensive. The trade-off is that the cabins are not as light and airy as the cabins on Dolphin Deck.
Another alternative would be Caribe Deck. The Caribe Deck cabins are not as large as the mini-suites; but the balconies are about the same size, and they are partially covered by the balconies above on Baja Deck.