Your goal for a swimming pond is to keep as many nutrients out of the pond as possible. That means these things if you can:
When the pond is built, don't line the bowl with any topsoil. Stick with subsoil or clay.
Make sure trees are nowhere near the pond as to not fill it with leaves. If leaves get in, get them back out if possible.
Do not fertilize near the pond. Make sure no runoff can enter the pond which may contain fertilizers or sources of nutrients (or pesticides for that matter). This may mean diverting water away if possible.
Don't plan on feeding fish, though you will definitely want some in the pond to control the bugs and snails. (Yellow perch, fathead minnows)
I'm not certain about aeration. Having that will keep the pond a lot colder near the surface delaying the start of the swimming season. Especially in NH. Its primary purpose in your case would be to keep organics from accumulating on the bottom. However it has drawbacks of high cost, keeps "stuff" suspended in the water column reducing clarity, and IMHO annoying to swim with.
Pond dye in a swimming pond will be effective at preventing vegetation on the bottom, but remember that it does not reduce nutrient load. You have to stay on top of dye since if you slip for a while, those nutrients will be put to use very quickly and you could have an algae issue to contend with once it starts. Instead you can have the option of planting desirable water plants to capture the nutrients before they are converted to less desirable gunk, such as water lilies and so on. Also just a couple grass carp can keep the nutrients trapped in the fish rather than the water. Just don't over do the GC, you want a little vegetation to help with clarity.
FYI there are some great threads on beaches here, I would suggest taking a look into it as it is much easier to accomplish before a fill.
