I don't know of any cruise lines that allow "downline boarding" except MSC that does loops with multiple points of embarkation/debarkation. Though they generally force you to sail round-trips so you can only get off where you got on.

I don't think the cruise lines will let you simply get off the ship and then board at another port just because you want to. If you miss the ship you can join at the next port at your own cost. I am not sure if the ship will allow this as a prearranged situation because of passport/visa requirements. I'm not sure how customes and immigration works with cruise passengers. This could be costly since you may pay a fine and require last minute travel cost to the next port. You can look at the World Cruises where they sell individual segments. So you may not want to spend 135 days on a ship, but the ship will sell specific segments of that cruise that may be heavy port days. Some of the lines spend overnight in certain ports if that appeals.

A cruise vacation is very different than a land based one.

There are cruises where the boat it the primary experience and the ports are second. There are cruises where the destinations can be the focus and the ship is nothing but your moving hotel, with the tradeoff that you are sometimes not close to attractions / center of the town ( think many European as well Far East Ports ). Easiest example is cruises that stop in Italy where the port is hour or more away from the big cities and you only spend a day or most two there.

The nice thing on the higher ones is the options around on ship experience. I've had friends that did cruises and hated it, others that loved it. We sprinkle our vacations with a cruise every year or so, and for a while did them annually with the kiddos. The experience of the port is really about the tours you take and money you spend, the ship is the brand. Take a Viking River or Ocean or Disney will be very different than a Carnival or RCL or Princess.

Some places like Alaska are unique and a cruise is the only way to really do it.

The brand you pick to look at starts with do you fly business/first only, economy or some mix including economy plus ;-) Or what kind of hotel you stay at when you vacation, same with you eat at 25/person, 50/person or 100/person.

FWIW I've done a few cruises and lines; Viking (Ocean/River), Princess, Disney, Carnival, NCL

Happy research and consider looking at the forums at cruisecritic.com

Some Mediterranean cruises have multiple embarkation points. We started off in Barcelona and some got off and were replaced by new passengers in “Rome”. It doesn’t seem like the most efficient way to fill a ship, but maybe is done to fill otherwise empty beds. And that was 10 years ago, and maybe it’s no longer done.

I have read on CC that you have to petition the cruise line to get off before the end of a cruise. I don’t know about joining late.

MSC is the only line I know that does that, and not just in the Med. It actually works much better than having between 3000 and 5000 people all boarding at one location.

Some cruises start in the Caribbean. Our next one is out of San Juan. It gives us an opportunity to skip the standard Caribbean ports that get pretty tired after a single visit. And we will hit 6 ports in 7 days. I would bet you can get a pretty reasonable flight to SJU out of BOS and it’s not much longer than flying to MIA.

Similarly, we're starting even further south in Martinique in January. It'll be our first Caribbean cruise since 2005.

It's possible as long as cabotage laws (such as U.S. PVSA) are not infringed. You'll have to ask for cruise line permission in advance as well.

In the first instance, if the cruise is a closed-circuit r/t US cruise, absolutely not (as least for getting on the ship at any U.S. port). Basically, any cruise where you embark at one U.S. port and disembark at another is forbidden, unless the ship sails via somewhere far, far away.

I have heard (or know of) instances where it;s fine (with advance permission unless on a cruiseco-organised tour) to get off at one port and get on again at another, but these are not in the U.S.

I don't know if a cruise would let you board at a port along the way but I do know they will let you off early. On an HAL cruise from Southampton to Boston a few years ago we disembarked early in Halifax because it was easier to get home. We informed the crew of our intentions and it was not an issue. Getting off early did not reduce the cost so if you intend to join a cruise in progress expect to pay the full fare.

As log as it doesn't violate the PVSA o r cabotage laws of other countries. Many people who live on (southern) Vancouver Island who do Alaska cruises like the Seattle R/T option as they can get off at the PVSA-compliance port (Victoria). Makes it easier to get home vs. the R/T ferry (or air) to Vancouver.

Cruising is a very different mindset. I love it, been for weeks on end on cruise ships, but its different than a plane/hotel combination. The most important thing difference is that the cruise line controls everything and you have no say. You agree to this when booking. Ship misses a port because of x? bad luck. Your cabin sucks? Bad luck. Catch Noro because people dont was their hands? Bad luck.
You can easily get to NYC where lots of cruises start/end, but its mostly. New England and Carribean from there, plus the Transatlantics. I loved the New England cruise, lovely ports and a nice, older crowds. Carribean cruises can get boozy.... though our february one was also more quiet and nice. Pick your ship wisely, this is where a good travel agent really shines. They ask what type of vacaction person you are and can suggest what ships would be a good fit and what not. We for example hate the large amusement park style ships, but others go nuts for them.

Lots of digital nomads on those btw, the internet while you are at sea sucks, even with Starlink, but if you can work without strong Internet its perfect.

I'm aware of that and I'm going to prepare for it! I know that ships may miss ports due to weather (not very much different from flights having to divert due to weather except I just don't get to go to that particular port). My cabin sucks? My hotel stays tend to be of the nonrefundable prepaid variety and I've definitely picked some hotels that I've sucked in the past. I just deal with it knowing that it's temporary! As far a noro, that's not something that I can control so I'm not going to be upset about it.

I definitely want my first cruise to either be Caribbean or Canada. Once I get my PTO at the beginning of the next year, I'll start planning and/or jump right in!

You actually can. We've been on noro ships, but have never caught it. Wash your hands frequently (hand sanitizer is not effective against noro) and don't touch the serving utensils, handrails, or elevator buttons with your bare hands.

Most cruise lines allow downlining
 

It usually must be arranged well in advance with the cruise company as they must get approval for the ports in question, as well as making sure that the itinerary does not violate any laws.

When not arranged in advance, it happens most often when someone misses their departure because they did not come in a day or two early and their flights were delayed or canceled. In this scenario, if the cruise line can help you board at the next port, they will . However the expenses of getting to the next port are on you. Unless you have insurance that covers your particular situation.

Happened to me and a travel companion this past July in Italy, Had to get to the next port on my own. We walked up to the ship, were taken to guest services by one of the sailors manning the gangway, and they signed us in with no hassle. We weren't the first people they had done this for, and knew we wouldn't be the last. It was all in a day's work for them.

We were fortunate that the ship's departure point and its next port of call were both on the Italian mainland and connected by frequent, fast, and reasonably priced trains, Could have been worse if one or the other was on an island somewhere. (We had travel insurance, but seeing if we might collect something from it wasn't worth the effort.)