Carp often have specific areas they hole up in winter, year after year. It could be silt beds, near gravel bars, natural foods like bloodworm beds, or even next to snags, decaying lily pads or rushes. It is most definitely not always the deepest water, I have seen them in rush beds with only just around 3feet of water above them. They were only visible if you got into the water right next to them, you couldn't see them from any swim, although you could cast to the edge of the rush bed, and catch.
In winter I rarely fish with any particles, maybe some very over wet soaked Vitalin as groundbait and nearly all boilie as my feed, although sweetcorn can produce a fish or two.
I also reduce my stringers; instead of 5-30 bait stringers, I go to only 1 or 2 (14mm) boilies on a stringer with a single hookbait. One rod nearly always has a hi-viz or hi-attract pop-up as the bait, and I recast this regularly, trying various spots and various baits. The obvious bright yellow Pineapple (in its various forms from different manufacturers) works sometimes, as do others, Monster Squid, Scopex, Plum or my own version pop-up Garlic and Megaspice, which I describe in the Bait Making section. I really don't know what it is, but that mix produces a lot of fish as an individual pop-up, or as part of a food bait mix.
Even in winter, the food bait will often work. I have described in the past a session I had years ago on Thwaite. For the first two days the high attract pop-ups produced, then for the rest of the week all takes came to the food bait I had been feeding sparingly since the first day of the trip after no other prebaiting through the year.
On Earith back all those years ago I found a swim that produced fish every trip, although because I was catching other people did start fishing it. A little digging around with the marker rod, I found a swim with similar topography at the other end of the island. I went on to catch from here as well.
My pre-baiting on here was as I left every week after a two day trip. I put a kilogramme of boilies in as I was leaving. Every take came on my good source bait. Despite trying a few times, the high attract baits did not give a fish. I wish I had known about the Garlic Spice mix then!
NOT all takes will come during the day. As much as fish may be caught during daylight, I found some fish still fed at night. I have caught a lot of fish at night, even in winter. My biggest common at the time, from Earith, was caught at around 1am in February. Saying that though, my best winter session 4 fish, over 4 days, just after a thaw, every fish came during daylight. The only reason I fished at night was the distance from home.
Earith Winter February 25lb common: