Drinks - for alcoholic plan for similar prices to on land, sodas and coffee, bottled water

if you want to do specialty restaurants, plan 20-50 per person depending on the restaurant.

gratuities - $12-15 per day. Can be prepaid.

Excursions - search for independent excursions to save money, will also typically be less crowded

Spa treatments - more expensive than land

Photos - can be nice to get