All of the previous comments are good. Now, let me summarize some of my points of view.
1) The bog should really have "pea" gravel for proper filtration and be lined with good pond liner material. Swimming pools use sand!
2) The bog needs to be deep, not long and should equal about 10% of the pond's volume. Water is usually pumped into the bottom of the bog through say 3/4" PVC piping constructed in a "H" pattern, center fed from the pump with each leg having numerous 1/4" holes (drilled through both sides) spaced every 3 to 6 inches. The "H" distribution piping is covered by 4" corrugated PVC drain pipe cut horizontally in half, so each leg is covered. Next cover the corrugated piping with about 12 to 18" of the pea gravel. Cover this layer of pea gravel with 1" thick heavy filtration media. Then fill the rest of the bog up with the pea gravel. The bog should have a discharge chute that is below the top of the liner and discharges into your pond.
3) The reason for the filter media is to reduce the possibility of any bog plants roots from working their way down and clogging up the distribution piping.
4) For reference, my bog is 24" x 72" x 48" deep and filters both a 1,500 gal and a 1,000 gal pond with lots of fish.
5) Depending on how much filtration you need to do, which would be fairly minimal if your pond is lined (which I highly recommend) but could be substantial if not, you will need to periodically backwash the bog. To do this, I connect a separate pump to the supply pipe to the bog and suck the water out from the bottom, which adding water to the top with a garden hose. I do this until the water discharge become clear. In my situation, my pond has a lot of fish in it, so I usually backwash the bog ever 2-3 years.
6) Bogs are great for fish ponds as they develop good bacteria that digest the fish's waste and keep the pond water crystal clear all year long. But in your case, with no fish and just a swimming hole, you will need chlorination assuming you don't want to get sick from eventually polluted water, hence reason #1 to fully line your pond. Reason #2 is that an unlined chlorinated pond will quite possible leach into the water table which I dare say that the EPA won't appreciate.
7) In constructing your pond. your really need to support the top 2-3 feet of the perimeter, preferable with concrete architectural block (shaped like a 6 sided diamond) and readily available at Lowe's or HD. Failure to adequately support the upper perimeter will most likely result in gradual cave-ins. Also, tuck in the pond liner half way up this perimeter wall from the inside to the outside leaving the top several rows of block exposed. Grade the surround landscape to just about 1/2 below the top course of blocks. Use a skimmer on a side opposite of the bog's discharge chute to allow natural circulation through the pond.
8) Finally,depending on the elevation of the bog in relation to the pond, I highly recommend installing a check valve on the pipe feeding the bog to prevent dirty water from draining back into the pond if and when the pump is off line.
Hope this helps and good luck!
