Winter can be a challenging season for farmers, with Crops, Grass, and Weeds all dying out at the start of the season, and with no highly profitable crops available. This an excellent season for Farm upgrades, The Mine/Skull Cave exploration, Tool upgrades, and fishing. Due to the lack of grass and weeds, Fiber and Hay is more difficult to obtain outdoors. However fiber still can be found in The Mines and Mutant Bug Lair, grown with Fiber Seeds, and is a byproduct of Wheat (grown in the Greenhouse). While at the same time hay can be purchased at Marnie's Ranch. This season has the most opportunities to obtain valuable items than others.
The Nautilus Shell can be found at The Beach only in Winter. In addition, the items shown below can be found while foraging during Winter. The percentages provided for each location are the percentage of all foraged items in the location that will (on average) be the specified item. 
These fish can only be caught in the submarine ride at the Night Market during Winter 15-17, or by using Magic Bait in the South-western corner of the beach. They can be caught during any weather, and at any time while the market is open (5pm – 2am).
↑The foraged items that can spawn in a given location each season are determined by GameLocation::spawnObjects in the game code, using input from the data file Content\Locations.xnb. The percentages provided here have been normalized to ensure that the percentages sum to 100% for each location. The data was normalized by:
Calculating the sum of all percentages listed in Locations.xnb for Winter
Dividing each percentage by the sum
↑ 2.02.12.2For all submarine-specific fish, the input data in Fish.xnb specifies upper/lower size values of 8 and 25. However, these fish can only be caught in the Submarine, where the maximum possible Fishing Zone is 2. As a result, even with a Fishing skill of 14 and a perfect catch, the largest possible fish size is 20 inches (which happens to be just barely large enough to be iridium quality).