There are three types of Ponds: purple, green and blue. Purple Ponds appear in Marshes and spawn Mosquitoes at dusk and night. Green Ponds appear in Grasslands and Forests and spawn Frogs during the day. Blue Ponds are found in Ruins and contains Eels instead of Fish.
Using a Fishing Rod, Fish (in surface Ponds) or Eels (in Ruins Ponds) can be caught at any time of day. Fishing takes 4 seconds plus 4 seconds for each missing fish in the pond. All variations of Ponds contain up to 10 fishes/eels, and one fish/eel is regenerated every 100 seconds.
Ponds will freeze in winter once the temperature is low enough, and when frozen, they won't spawn creatures. They also cannot be fished, though they can be walked over.
Ponds spawn with 2-4 Plants along their banks, which can be examined separately. They cannot be picked, but they can be set on Fire to yield Ashes. The plants will grow back every Summer. Ponds found in the Ruins are surrounded by Cave Lichen fungii that resemble Lichen instead of the normal plants.
When fishing, one should be wary of Frogs and Mosquitoes, as they are aggressive.
When fishing in a Marsh, it should be noted that the Marsh is also the native habitat of Tentacles and Merms.
One may fish in Frog Ponds unbothered at dusk when the Frogs return to the water, and in the spring and fall when it is warm enough to thaw the Pond but not warm enough for Frogs to spawn. One may fish in Mosquito Ponds unbothered during the day, since Mosquitoes only come out at dusk/night.
Frog Ponds will sometimes show up in a small cluster. Pig houses can be built near/inside this cluster (preferably walled off) and the frogs will kill any pigs that spawn during the day but will not eat the meat. With this setup, Pig Skin and Meat can be collected regularly at dusk/night.