Most people who go out with tour companies have no experience with kayaking of any kind. The guide will give you a lesson before starting out on the trip.
White water kayaks do not have rudders, while most sea kayaks do. Sea kayaks tend to ride over most waves while white water kayaks tend to dive through the waves. You do not get very wet in a sea kayak which is an important consideration when paddling in the chilly waters of Maine.
You just need to be in average condition to enjoy a few hours of paddling. A good tour guide will adjust the pace and the distance of the tour to the skill of the group.
I recommend a light pair of cotton trousers (shorts are fine on warm days). Blue jeans are not recommended. Bring something to keep your upper body warm in case it gets breezy. If it looks like rain, bring some rain gear. Polypropylene and polar fleece garments are great for cool to cold days on the water. Sneakers or sandals are OK (they will get wet). You can ask questions when you make your reservation.
For more information, see "What to wear and bring."
Bring along plenty of water or juice to drink. Even on a short
trip, you may wish to bring a snack for that added energy. If
you're out over lunch hour, you'll definitely want to bring some
nourishing food along. Sun screen, sun glasses, and a hat are
a definite recommendation
There will be wonderful picture opportunities when you're kayaking. However, a disposable camera might be the best idea. If you do decide to bring expensive camera equipment, you do so at your own risk. Wrapping your camera equipment in plastic bags before you place it in the dry bag will give it some added protection. A strap on your camera is a good safety feature. I've seen amazing juggling acts just before a camera was committed to the briny deep, that is dropped overboard.
As the saying goes: "If you don't like the weather in Maine, wait a minute." Weather often varies significantly in different parts of this small island. Morning fog is quite common,but often burns off as the day progresses. Tours are conducted in conditions of fog or light rain. Thunder storms seem to be rarer here than in other parts of New England, but they do occur. Fortunately, they can usually be seen coming in many of the places that the tour groups paddle. Five to fifteen knot winds are common, but I personally do not like to paddle when the wind is going to be twenty knots or above.