Now you're talkin' ...this is what I do everyday, well at least until August 1st when I transfer.
First you have to identify what is you're really wanting to do and what you need the boat for...
If this is a "seasonal" thing...perhaps you're best bet is to join one of those clubs where for a nominal fee, you have access to a boat in the same manner as one would utilize a time-share property. Buying a boat is rather large investment and it's best to "test the waters" if you may, before making the jump to own a boat.
But if you're really looking to buy a boat...then...
I would recommend buying a 25' to 36' GRP (fiberglass). The final length depends on how far offshore you're looking to go...obviously the further offshore, the slighter larger vessel would be the better choice. It has a lot to do with center of buoyancy, center of gravity, righting arm and roll period. A GRP hull is extremely buoyant...buoyancy is the effect of the water pushing UP on the hull and the gravity is the force pushing down on the hull. When these two forces interact, it causes the boat to roll side to side. How fast the boat rolls from left to right, then return to even keel is called your roll period. In most cases, a longer boat means a wider beam (width) and the wider the beam, the longer the roll period. A shorter roll period means the boat is going to snap back to it's trimmed position and a day of this jerking motion makes for a bad trip.
Also, the advantage of GRP is maintenance. Fiberglass is easy to repair when compared to wood or aluminum...no different than repair a surfboard...laying on the strips of glass fiber and coating with resin - pretty basic stuff. An aluminum hull you have to gas-free (for gasoline engine w/ installed tanks) or you have to remove foam in the welding area. Wood - that a can of worms I don't recommend going near...marine grade, treated wood is expensive and the skill required to make repairs is hard to come by, not to mention labor-intensive.
Ok...lesson in hull design is over...now for operation.
Obviously I'm partial to vessels operating under power...I hate sailboats. Sailboats are great if you have the time to learn, but it takes a while to fully master the skill. Being on the water during peak boating season is not the time for hands-on training - proficiency is key. Boats aren't like cars, there are no brakes, and that is most obvious with a sailboat.
For basic boat operations, recommend taking a Coast Guard Auxiliary Boating Course...they will teach you everything you need to know about required safety gear, navigation, radio communications, and they'll even inspect your vessel for compliance with all applicable federal regulations. Many states offer the same types of courses through their respective Department of Natural Resources offices.
Costs (I'm going to lay out what I have to contend with on a daily basis):
1. The boat itself
2. Registration (State)/documentation (Federal) This depends on the size of the boat
3. Insurance
4. Towboat U.S./SeaTow/ Boat U.S. etc...$100 or so (annual membership) and they'll come getcha if you breakdown and need a tow...without it, last I checked, it was on average $60.00 an hour, plus fuel, and a $300 charge once lines are made fast to your boat...Belive me when I say get the insurance - I been on towing cases that have last 14 hours. Towing takes a long time...first they have to get on scene, then hook up tow, then tow you back to the dock. Towing speed is brutally slow and is figured as follows: (Square root of waterline length) X 1.06 - 10% for safety...so, for example a 28' boat with a 25' waterline has a safe towing speed of approximately 5 knots, or 5 nautical miles per hour - if you run out 10 miles and then north 30 miles, you're looking at 8 hours of towing back, not to mention search time and fuel.
5. Boat trailer
6. Spare parts: Spark plugs, filters, light bulbs, spare parts for the trailer (bearing buddies, etc)
7. Boat outfit: anchor, anchor line, mooring line, fire extinguisher, paddles (no, you're not going to get far paddling a 30' boat - but they make great depth indicators when you're running in unfamiliar or unmarked territory. And if you do find yourself aground, you can push yourself off).
8. Electronics: I am a big believer in a DSC (Digital Selective Calling) VHF-FM radio...first of all, don't think you're ok with just a cell phone, the minute you go offshore, you lose coverage - invest in a VHF-FM w/ DSC...AND REGISTER THE DSC...there's the equivalent of a panic button on the radio that you can push and you're REGISTERED boat name or hull number is passed to CG ground station along with your latitude and longitude in the event you find yourself in trouble. While I'm on electronics...they're a bit pricey, but if you're going to island-hop - invest in an EPIRB, but make sure you register it.
9. Safety gear: CG-approved Life jackets, Type IV throwable PFD, soundproducing devise, flares if you're offshore, etc.
10. If you decide to buy a used boat, recommend hiring a marine surveyor - same process as a home inspection. I've seen many cases where a boat split in half because the hull was dry-rotted and the new owner claims "I just bought it last week."
I see someone already answered your dock space/slip fee question, or at least provided a link...those vary by location so it's going to take a little planning.
CHAPMAN's PILOTING...excellent book. First book I bought when I returned from CG Basic Training and still use to this day even though I'm a LT...excellent resource, especially for the beginning boater.
It also wouldn't hurt to find a copy of NAVRULs...There's plenty online...Boat US even offers an online NAVRULs test.
http://www.navcen.uscg.gov/mwv/navrules/download.htm
Another great resource that we use to teach our new CG crewmembers the "essentials" is the Boat Crew Seamanship Manual:
http://www.uscg.mil/ccs/cit/cim/directives/CIM/CIM_16114_5C.pdf
This is by no means an all-inclusive list... but hope some of my info helped...I could go for about a week about buying a boat, so if you have a specific question, feel free to ask or IM or shoot up a flare, etc.
An remember...if you get in trouble out there...VHF-FM Channel 16 is your INTERNATIONAL DISTRESS and HAILING freq...and remember the 3 P's!!!
People (number passengers on boat)
Position (Latitude/Longitude)
PFD's (put them on!!)