When it comes to diamonds they are great at removing metal fast but most people will tell you if they really know anything about sharpening that you should finish sharpening with a wet stone of some sort.
I prefer to finish with water stones after I'm done using diamonds,I find I get a better longer lasting edge finishing with wet stones,you don't need to have a super polished edge either for it to last a good while.
But then there's other thing's to consider like what blade steel are your knifes made of,I find M4 M390 S35vn and S30v S90v and S110v are all great steels and there are many more that great at holding and edge,you will find M390 and M4 get extremely sharp and are both very aggressive cutting steels with a polished edge,but you will also find that S110v is better with a toothy edge finished anywhere between 400 and 1500 grit max and if you polish it's edge it's the biggest of time you will have ever spent in your life but it's not very sharp with a polished edge and the edge will dull very fast unless it's sharpened with a toothy edge and I still use diamonds on S110v and then finish with water stones to refine the edge.
Another factor is how good was the maker of your knife at heat treating the blade,I know Mike Emler who sharpen's a lot was saying something about a company that use's S35vn and the sharpness and edge retention of their S35vn was way better then any other S35vn blade he had ever had and said that company also had a special way they heat treated that steel and really had it figured out.
If you mention the knife's and blade steel's they have that will help to,if you just have cheap knife's that are 20 to 50 dollar knife's your most likely never going to have good edge retention if that's what your after.
For water stones I really like the Sigma Power stones in the finer grit's you can get them from Jende Industries and I think they will sell them unmounted if you ask for them that way and they may even mount them for you if you ask them to,I like to use a rust eraser that you get from ebay in medium grit and and they are grey rubber with a bit of abrasive in them and I rub my water stones with them to make a slurry and that help's to get the stone to work faster for you more so in the really fine grit's.
If you do get water stones get bar keepers friend in the powder version to clean the stones trust me comet and the other powdered cleans won't do crap no matter how long you try with them,with bar keepers friend the swarf or metal filing's whatever you want to call them just lift's right out instantly and you don't need to use much at all.