We sailed with our 18 month old daughter (she is now four). Yes, we used the nursery and she was very happy there. We didn't bring any of her toys with us so the only place she could explore and play with toys was the nursery. We found (for us) that putting her in the nursery during dinners worked the best. The dinners are long and the dining rooms are busy. She was happier eating in the nursery, we were happier because she was happier so it worked out. Our stateroom host put a rail on the bottom bunk and that worked well. There is milk in the free beverage stations and that worked out well too. I just served the milk into one of Disney's cups and then poured it into her sippy cup. We did bring a small stroller and only used it when we went on port excursions AND to carry the wine on board.
She did really well and liked the water play area; she did not seem to notice that she could not go in the pools and I was relieved about that. I had to bring a lot of diapers! By the time we drove up to our house after the trip, I had exactly one left in the diaper bag but the additional space was great for souvenirs. We have just returned from our second cruise. It was easier since she was four but again we had a fantastic time with her at 18 months; we just had more freedom to do things together as a family since she was older and also more time as a couple since she loved the kids' clubs.