You don't need a kit. You can put together a few things pretty cheaply, and then get the supplies in bulk. While I haven't yet brewed on a boat, I will be doing both mead (honey wine) and a couple of types of easy beer (beers that don't require special supplies).
So, while we're preparing for the next couple of years to get ready for our trips, we're working on what supplies and things we need.
I've been making homebrew a few years now - and mead, and I started using the little "kits" like "Mr. Beer" and some of the others.
They... to be honest, suck. THe beer is not that good.
You can do a much better job with a simple 6 gallon bucket (get one that is meant for food and better to purchase it through an online brew store or a local place in a major city).
Get yourself a couple of recipes for the types of beer you like - and talk to someone in the local area ( I strongly suggest going to a home brewing store personally and talk to them) and get as much information about supplies, prices and the process you can.
Many will actually take you through the process (you brew a batch of beer on the premises for a fee) and they will take you through the whole process.
Making an All-Grain batch can take a lot of time and you will be cooking your wort (that's the stuff the beer is, before it is fermented) for as long as 2-3 hours, depending on the types of beers.
I would suggest looking into what we call "Extract Brewing" or "partial grain" brewing.
This is where you obtain small quantities of the grains you will use in your recipe and then add a malt extract to fill out the majority of the fermentables (sugars). Malt extract is nothing more than the allready-cooked-down stuff you are going to be getting out of the grains. In smaller, extract batches you use the grains for the flavors and colors mostly.
Also, a good ale yeast (pick one or two, and stick with one of them!)
You're not probably going to be making lagers on a boat without cooling systems and such, so pick a couple of ALES you like and go from there.
You want this book, or one of the editions by Charlie:
Amazon.com: The New Complete Joy of Home Brewing: Charlie Papazian: Books
Welcome to HomebrewAdventures.com - News
Check out that site.
While you're at it, check into the American Homebrewers Association. I joined... I think it's 40 bucks a year or something for a family membership. You get between 10-20% off meals and beer in a LOT Of restrarunts. (I visit Rock Bottom a lot and try different beers there, know the head Brewer here in my hometown and get recipes from him, and he's done a couple of ours!) I figured 4-5 visits to places usually pays for the membership in the savings
Also a lot of homebrew shops will honor your membership and give you discounts as well.
Now, about that "constant agitation"... I don't think it will hurt things a bit
A basic process goes like this:
1) Clean things up, prep for cooking.
2) following cooking directions prepare your wort and cool it.
3) After cooling, pitch in your yeast.
4) Wait - a few days. Usually 3-6 days depending on the wort, amounts of sugars in the wort, the yeast and the temps.
5) Rack the beer (put it into another container)
6) add some bottling sugars (3/4 cup for a standard 5 gallon batch)
7) bottle (you need some tubing and other things, look those up on a brewing site, and check out that book which takes you through the process more carefully and in detail)
8) Wait some more. Usually 1-2 weeks for the beer to finish fermentation in the bottle (what gives it fizz and puts the co2 in it)
9) Chill and drink
I promise you, going that route instead of a kit (OR buying commerical beer!) is going to be much better in the long run. You control your process, you can make just the beer or beers you like, in quantities you like and can handle and home made beer, if you do it right, JUST TASTES better!
Good luck. Send me a PM if you want more information.