I used to a own a boat that was identical to yours, 2001 Four Winns 180. I put a custom made box to fit under the rear seat in the middle, it housed a jl audio 10w3 and I pushed it was a fosgate 250.2 amp.
It didnt sound good because it rattled everything in the boat. The sub sounded really good in my truck, but was unbearable in the boat. Whatever sub you buy, the biggest decision will be finding a solid place to mount it.
For a sub however, I would probably go with an infinity basslink. They sound good already come with built in amplification, come in many boats as factory options.
Here is one for $180, buy-it now. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=32822&item=5705394 559&rd=1
If you dont like it, you can always resell it or put it your truck during the winter months,