Re: what to look for
Your requirements and limits conflict. I am assuming you also have little or no experience.
First, safety. If you are boating in the coastal regions of Boston, forget deck boats, pontoons or typical bowriders. They are for lakes. You need a seaworthy hull, and a dry design. See "capacity" below.
Second, manageability. Something that sleeps 5 and carries 9 will not be a trailer boat. If you insist on trailering you need serious towing capacity, serious ($$$) storage and hauling and ramp skills.
Third, size v. water sports. You can tube behind an aircraft carrier. But anything over 22' is not suited for water sports--not only will you go broke on gas, it's not manueverable enough for fast starts and turn-arounds to pick up the fallen. And since watersports are usually done in small bodies of water (to get smooth water) I think it is dangerous to operate large vessels in close quarters when (typically) others are using it, too. Now, if you just want to drag the 6 year old around in the tube, fine. (Yes I know lots of people do watersports behind big boats, just my opinion).
Fourth, sleeping. Take a hard look at this one. For the typical family, you would seldom sleep on the boat. If you do, you are into "cruising" which is very different than the Saturday boater. You need a big boat and big bucks. You would save money in the long run just taking the open boat to a marina destination and getting a room there. So start without the sleeping quarters and move up. And if your girls are the type who can manage camping, you don't need a head in a cabin. Avoid the cabin and you have flexibility.
Fifth, capacity. You need lots of open space. A boat rated for 9 is comfortable for 6. Factor in how many passengers are small children, too. (Realistically are you taking that many people each time?) Maximize open space by design. Outboard motor, no wrap -around dashboards and sweeping windshields and sofa seating. Now, a 24' carolina Skiff is rated for something ridiculous like 15 but you have ample room in it. Look at them and also at Sea Chaser for an inexpensive, light small cabin boat with lots of room--and it's seaworthy. Look at center consoles with built-in bow seating--sea hunt, contender, regulator or dual console key west, sea hunt, etc.
Sixth, budget. If you have $20,000 to spend, your boat can't be more than $18,000, plus you need to be able to put $2000 in for repairs at any time. The older the boat, the more you need for start-up and reserve. In your case, you need to spend more at the front end and buy reliability. That means a newer boat--and therefore a smaller boat.
Seventh, timing. Buy a used boat this week and you won't be on the water until July 15. Buy one that needs a little work, and it will be August 1 at the earliest. Short season in Mass. unless you are hearty and have a dry boat!
How to shop--go look at new boats at dealers and on line to get your size and style. Take your wife and walk around the marinas and boat lots. Only then start looking for used ones that match your needs and ability.
I would start you in a fiour year old 19' Grady center console with a 150 if I had the magic wand. That's what I steered my sister's family to. Worked out great. Or a 21' carolina Skiff for maximum bang for the buck.
