It is true, water is a big part of a ducks life. I set up a kiddy pool on the
lawn in the summer, I dump it almost daily as fertilized lawn water saving me watering by hand. As they say the devil is in the details.... when I dump the water the ducks cannot be around until it is soaked in or they will damage the lawn looking for worms in the soften soil. It only take a minute to soak in though.
In the winter I skip the pool, living in Oregon several puddles show up all winter long and that's
enough 'new water' to keep them until summer. I always have a couple of buckets of clean water around for drinking and washing their heads in (very important for the health of ducks). Julie is right, for a dozen ducks you'll want a couple of pools, placed away from their hang out area, this way they will forage for bugs on route to the water and back. Make sure you have fresh, unfrozen buckets of water in the winter, plus swales/puddles in the spring and fall and your ducks will do fine.
I really want to raise a few ducks because I have heard they will eat insects in the garden with less destruction to the garden than chickens usually do. Is that true?
Yes. {edit} Ooops I forgot, ducks will run-over any pants just coming up. They have a blind spot in front of them.
I'd say no plant shorter than neck level. And they will nibble some veggies plants down, if the plants are small this will take them out.
So timing is everything. I set up semi circle wire around new spring plants in the path of the ducks just so they can grow getting big enough to body-block the duck.
I let the ducks into the garden area all spring to overwhelm the new crop of bugs. Once I plant the seeds or seedlings the ducks run the perimeter outside the garden
fence. Once the veggies are as big as the ducks they are allowed back in for short visits, or over night (ducks are calmer, forage less at night).
I am leaning toward the Khaki Campbells because they are supposed to be really good layers, but what other breeds are hardy, lay well and do a good job at bug control.
Khaki's are half Indiana Runner and half Mallard and can breed with both, not all breeds can interbreed. Indian Runners are skinny and not much good for meat, but they are the best egg layers and bug eaters, this is where the Campbells get it. The Mallard's are meatier, good moms, but they love to fly (away?) and not so great at laying. This is why Campbells are popular - meat plus laying and they will raise young three out of four seasons if the conditions are right. I keep Runners, which only sit a nest once a year in the early spring, and are not the best attentive mothers - but this works great when egg collecting is what you want.
I Have no idea about blister beetles. However I bet you could raise ducklings to like the taste by feeding smashed ones from the get go, creating your own blister beetle eaters. It would be a fun experiment anyway.
I want to build a pond anyway, but may not have it right away, so would be interested in hearing your experiences of ducks with and without a pool/pond.
They will muck any water, because they constantly clean mud from their bills and nose holes. I would say not to provide anything of an in between size (horse trough, etc.), either keep it small and dump-able (kiddy pool) or larger, self cleaning (non-stagnate pond), with reeds, water movement, water flow in flow out.... etc. However, if it freezes where you live the pond won't be usable in the winter anyway so no rush on that
If you do not get winter rain then you will need to provide some source of unfrozen water. It doesn't have to be much, just enough to completely dunk their heads.
Last of all, ducks tend to mate in water, or at least that's what they want to do. So if your hoping for eggs to hatch you will need to provide water they can stand in, even if that's only a puddle or inverted trash can lid w/ water.