Don't think you need to buy gear if its your first trip. Also depends on where you are, what kinda water, what kind of boat, whats the weather?
I mean as far as clothes to wear. I'm up on the North Shore of Massachusetts. It's a single person Kayak and it's going to be in May. Not sure if that helps but I'm just trying to share all the information I have as of now.
Synthetic fibers are what you want for your clothes, since they'll dry out much faster than cotton. Look for a camping goods store like REI and ask the staff there what they'd recommend; someone there might have already been to where you're going. (Edit: I did a quick search, and it looks like your best bets will be the REI store in Reading or the North Face in Peabody)
Sneakers are not a good idea. It sounds like this will be ocean kayaking, and salt water can utterly wreck stuff. If this is a river, sneakers are still not a good idea except for when you're in camp. I'd go with sandals, preferably those of the closed-toe variety (they make them, and they're great).
Are you doing this trip with an outfitter or someone else who will supply gear--life vest, helmet, etc.?
Thanks for the info Thomas. I actually might try the EMS in Salem, NH as I'm right on the boarder. I'm doing the trip with a place that provides all the gear I just really need to dress for the occasion and bring some snack bars.
Right, then--synthetics or wool, as Mr. Robinson suggested, closed-toe sandals, a hat, sunglasses, sunscreen, something warm and comfortable you can wear around camp, and at least one quart-sized water bottle so you can hydrate yourself aggressively.
A good, properly sized PFD (personal flotation device, aka life jacket) is key. If you're going with a guide or a friend, you'll likely be able to rent or borrow. If not, I'd suggest checking out REI, Eastern Mountain Sports, LL Bean, and maybe West Marine. Tell the staff what you're looking for, try on a few, see what you like, and then shop around online...you'll almost certainly be able to find a better price. I have a Stohlquist vest that I've had for 6 or 7 years now that's held up great.
As already mentioned, synthetics are the way to go. In May the weather is generally warm and the water generally still pretty cold. If your kayak is self-bailing (holes in the bottom to drain water) you'll be sitting in water all day, so a wet or dry suit or bottom wouldn't be totally out of the question. Layering will be important as well...a synthetic base layer like under armour, synthetic pants / shorts and shirt, and a synthetic layer for insulation like fleece. A spray top or rain jacket is also a good idea, as it can keep you dry and will also block wind.
Sun protection is going to be very important as well. Long sleeves are a good idea, and a synthetic shirt with built-in spf is even better. Sunglasses and sunscreen are a must, as is a hat. Bring plenty of water, any required meds, basic first aid kit and some high-energy food, a charged cell phone in a waterproof case, a whistle, a mirror, and a knife.
I have no problem kayaking in sneakers, but remember that if they get wet they'll stay wet...and forget about wearing good shoes, the second they touch salt water they will be unstoppably rank forever. I prefer Tevas or other synthetic sandals. If it's cold you can always throw on some wool socks.
My dad is an insane veteran Kayaker. Builds greenland-style boats, ocean kayaking, year round, the whole nine yards.
His standard summer apparel is an underamor long sleeve thermal shirt, crocs and board shorts. You'll probably want gloves if your hands arent calloused, and you'll probably need a life preserver as well if one isn't provided.
As someone who doesn't have my dad's tanning genes, I'd also recommend a bucket hat.