They use so much water and water is expensive here
Wrong. They use less water than washing up by hand. Ours uses 11 litres per cycle. That's about half a sink full. Could you really wash up 10-12 plates, crockery, pans, cups and glasses, breakfast things, stirring spoons etc, in half a sink full of water? No way.
How much a dishwasher is useful is more to do with how you cook than family size. It takes the same number of pans and roasting trays etc to cook a meal for 2 as it does for a family.
There's 2 of us (and 4+ cats so their bowls go in too) and because we cook most meals from scratch it goes on nearly every day. A family that lived on mostly convenience food or ate out a lot probably wouldn't produce as much washing up.
OP definitely get one if you want one - we usually use Aldi own brand all in one tablets and they're good. Or branded like Fairy if I see a good offer, but I don't think the expensive branded ones are any better - I aim to pay no more than about 10 p per tablet. Although if you have hard water, it might be better to use separate detergent, rinse and salt so you can put more salt in. Again, supermarket own brands are fine.