I'll second cnnashman. Maine ocean temperatures in December are in the 40-45 degree range. This is seriously deadly dangerous stuff! An inadvertent dunking (which can happen in lots of ways) or a sudden storm will leave you instantly at peril if you are not prepared. First, abandon any thoughts of swimming to shore if you are unable to quickly re-enter your kayak. Swimming or any exertion pulls heat from your body much more quickly than simply floating, plus the shock of the cold will soon disable you, plus swimming with a PFD on is not very practical--and you're certainly not going to remove it, are you?
So you have to, #1, be dressed for immersion (and, ideally, for longer than you expect to be immersed), and #2 have a well-functioning and well-practiced method of re-entering your boat.
A true drysuit with proper underlayers is clearly the safest garb for your neck of the ocean. Lacking that, you need a FULL wetsuit. I have never used waders and, without wishing to sound disrespectful of other posters, I don't like the sound of them. The purpose of a well-fitting wetsuit is #1 to prevent flushing: yes, water enters the suit, but in a good suit it pretty much sits there next to the skin, where your body warms it and (for the #2 purpose of a wetsuit) where the neoprene insulates it regardless of being wet. In contrast, waders are just big buckets: they don't stop the water sloshing and flowing over your body and stealing away heat, and they don't provide any insulation themselves.
My second concern about waders is that they are big buckets (and fly fishermen and ocean clamdiggers do occasionally die as a result of that). If you are not wearing a semi-dry top, like Sherminator suggests, your waders, unlike rain pants, will fill up with water like buckets and the weight will make it even harder for you to reenter your boat (and once you get inside, the waders will still have a lot of water sloshing around, with no way to drain it). If you are wearing a semi-dry top and are in the water for a significant amount of time, say from shock or injury or difficulty reentering your boat, you will probably find out fairly soon why they are called semi-dry tops, i.e., they will leak around the waist when your waist is submerged, or, heaven forbid, will leak around the collar if your neck is submerged if you have a neoprene seal. Semi-dry tops are designed primarily for river kayaking, where if your garment leaks you have shore nearby all the time to bail you out. The best of them have fold-over rubber flaps to mate with spray skirts; you won't have that.
My other objection to the garments suggested here (except the drysuit) is that they all rely on fleece fleece fleece for warmth. Fleece is great under a drysuit, but once it's soaked, as in waders, it will provide very little insulation compared to neoprene (when backpackers say fleece and wool still keep you warm when wet in comparison to cotton, they don't mean when it's completely soaked, they only mean that they will hold less water than cotton when allowed to drain). Fleece worn under neoprene will only contribute to water flushing though your suit, since the fleece is porous and doesn't seal tightly against your skin. You don't hear about scuba divers wearing fleece under their wetsuits. If they need extra warmth, they go to a thicker neoprene or add a neoprene jacket. Ask the divers at your shop.
I suggested a FULL wetsuit (vs. farmer jane and jacket) because it is the most effective in preventing flushing. A farmer jane will definitely be better than waders, but not as good as a full suit. With a full suit you can (and will in Maine) add a jacket for additional neoprene warmth, but I doubt you'll put a jacket over a jacket over a farmer jane.
You are probably going to want at least a 4-mm full suit plus a 3 mm jacket plus paddling (Goretex-type) jacket and pants for Maine. I use a 3/2 most of the time (3 mm over torso and upper legs, 2 over lower legs and arms) with a 3 mm jacket added sometimes (plus paddling jacket and splash pants over that), but I wouldn't consider that adequate for temps in the low 40s like you'll have.
And don't forget your feet: you'll want insulating neoprene boots for sure. I wear Seal Skinz waterproof/breathable socks over wool socks; if you get the right model (I forget which it is) and use them carefully according to instructions and put the bottom of your wetsuit legs over them to help them seal, they will often stay dry inside full immersion, sometimes not; even if they don't keep all the water out, they do add warmth by reducing flushing.
A warm wool, or better yet neoprene, head covering will greatly increase your safety factor because it reduces the initial cold shock that can kill you by causing you to involuntarily inhale water, as cnnashman says. I've taken to wearing a rubber swimming pool cap for this purpose when in colder waters.
Three final notes:
1. If you do want to wear a rash guard under your wetsuit, polypropylene is far superior to the polyester most manufacturers now use. You won't notice while you're in the water, but you sure will when you get out of the water and in camp if you're camping (or when you get out of your suit if you never get dunked). Polypropylene absorbs a lot less water and therefore drains and dries out a LOT faster. Some manufacturers even use nylon, which is even worse than polyester. However, finding good polypropylene is difficult these days. Manufacturers switched to polyester because polypropylene had too many issues with odor retention (which can now be solved with fabric treatments, silver threads, soaking in enzyme solutions, e.g., Nature's Miracle, from the pet store, or use of the fabulous anti-microbial deodorant Lavilin).
2. On his epic kayak journey from Japan along the Kamchatka and Siberian coasts to Alaska (read "In the Wake of the Jomon"), Jon Turk and his companions used drysuits for the first half but then switched to wetsuits. Unfortunately, he didn't say why (maybe too fragile over such a long journey through remote country?).
3. When I meet people interested in serious sea kayaking (not just shoreline piddling), I always urge upon them as a starting point that they read "Sea Kayaker: Deep Trouble", by Broze and Gronseth, for accounts of some of the many ways you can die at sea. Not dressing for immersion is way up high on the list.
Be prepared, then go enjoy!