To go 20-miles out into the ocean for whale watching or angling with eight or more people you will need a lot more boat than a 210 MONTAUK.
I don't think you can find too many really nice used 210 MONTAUK boats with a good trailer for $40,000. In a simple cursory search I see 210 MONTAUK used boats going for $64,000 to $99,000. You might not be able to find any at less than $40,000. You should check the price of a new 210 MONTAUK, as they are now well above $125,000.
For long distance trailering any boat, you need a very reliable boat trailer and a very reliable truck.
Since you will be launching in saltwater, you really need a extremely well set-up tandem-axle trailer, probably aluminum, or perhaps galvanized steel, but never a painted steel trailer, with top-line disc brakes on all four wheels, everything possible using stainless steel, no leaf-spring axles, so torsion axles instead, with guide posts, a spare wheel-tire, and a really good bearing and sealing system. That is not something you will find under every used boat.
Boats on trailers with fuel in the tank, and a lot of added gear aboard always weight much more than you think. In
Boston Whaler's performance test data for a 210 MONTAUK the boat weighed 4,484-lbs. Subtract 400-lbs for the two guys abourd, and the boat weight was about 4,100-lbs. A typical steel trailer with a 5,200-lbs capacity will weight about 1,200-lbs. So your towed weight will be at least 5,300-lbs, and possible higher to account for any added gear on the boat.
Towing capacities are stated for a vehicle with only one person (the driver) and nothing else in the truck. The towing capacity must be reduced to compensate for every person and everything you put on the truck. So another adult and two kids will be maybe 300-lbs reduced towing. A lot of luggage, more reduced capacity. Also, be sure the truck has the towing package, which usually affects the rear axle gear ratio, adds a larger radiator, adds a transmission cooler and possible an oil cooler, and provides a drawbar factory hitch and trailer wiring.
As for long-distance (400-mile) trips, if you want to maintain highway speed, and if any grades are to be encountered, the truck tow rating should be considerable higher than the towed weight. For that 210 OUTRAGE, maybe 7,200-lbs might be about the minumum, as with let's say 500-lbs added people and gear, that rating is reduced to 6,700-lbs. Now you are towing 5,300-lbs with a truck rated for 6,700-lbs. Yes, that will work, but there is not very much margin.
Also, for towing, a longer wheel base adds stability. However, a truck with a long wheel base makes parking the truck when not towing more difficult, and a long truck just does not fit in more marked parking spaces.
If you can find an "older truck" that fits all the criteria and is "very reliable" and is modestly priced, you will need many circumstances to align for you.
As for being able to rent a truck to tow a boat, you should call some truck rental agencies and see if they allow that use. I am skeptical that you could rent a truck in either Connecticut or Maine, then drop it off at the other end of the trip, and be allowed to tow a boat with it.
NorthShoreWhaler wrote:Q5: have you thought about getting two smaller boats?
I hope the thread originator has NOT seriously thought about getting two boats. If this advice were followed, then everything is DOUBLED: twice the maintenance, twice the registration costs, twice the upkeep, twice the storage costs, and all that to save two long days of driving each year. That solution is not practical.