Constipation and dehydration are a special risk for those who are housebound or bedbound and don't remember to drink throughout the day on their own. If you need to ease constipation, remember that there are more sources of liquids than a glass of water. Varying the liquids you offer is a good way to encourage someone to drink more.
Some options to try:
Juice (especially prune or pulpy orange juice if the person is swallowing well)
Lemonade or waters splashed with citrus or other fruits
Soup (broth or cream-based)
Thin hot cereal (such as Cream of Wheat, diluted with milk)
Popsicles or juice pops
Melted ice cream
Avoid liquids containing caffeine (coffee, tea), which act as diuretics.