New washer/dryer that's easy to dis/reassemble??

That looks like your transom door?
The enclosing cabinet has to come apart. New bungs and re-finish to put it back together.
The aft door has to come off with the hinges.
You want a bunch of young skinny strong stupid yard kids to move the equipment.

What brand boat and have you talked with the factory yet?

Bungs on repairing the wood enclosing cabinet. You wont see any screw heads with wood bungs finished over them.
With the transom door off, the unit tilted long ways on it's side, should fit out between the transom door hinge eyes.

The cost of the new equipment is real cheap vs the cost of removal / re-install..
Ya sure you want a new washer?

Eager to hear what the factory says.

On a Hatteras, from low mid-ship, we had to wrap in moving blankets, wearing hip & shoulder slings, kept it perfectly vertical to make the spiral stair case, then laid down & out thru the wheel house, galley, saloon, rear saloon door, over the cockpit & over the transom.
It's in the owners hobby garage now.
New Bosch equipment (2 pieces) went back in after a new closet was made for them.

The Whirpool apartment sized old school stackable comes apart pretty easy. What about looking into new front loaders, most of them will stack with a stacking kit and they make counter depth ones. That being said, you might be better off pulling a windshield and getting it out that way.

How deep is it? Just to make sure?

Why are you planning to go to all this trouble given you said the existing machine still works? It doesn't look too bad from your photo. You can get appliance touch-up paint if needed. Your 20 year old machine will probably still outlast the new junk they build today. Besides, you can't hardly buy a washer/dryer today without waiting months to get it. You may wait even longer if you are particular about what make/model you want. I would spend my time and money elsewhere myself.

My Kenmore stacker is original to 1994 and still going strong (knock wood). The dryer is a little slow to get a big load of towels dry but I’m in no rush when I’m on the boat.

I assume you are talking about inside the dryer drum. If it's real bad, you can take it apart and remove the drum. The drum can be either replaced or repainted with the baked on finish by a shop. You would be taking apart a new one anyways.

I had similar constraints. Old one piece unit with out via sawzall. Bosch 300 Series in two pieces came back to replace. The 300 has a 23.5" dimension.

Inside the drum? Who cares what the paint looks like, as long as it's not rusty.

Our 1988 Viking has the Kenmore stackables too and we use them regularly. The dryer lint screen was not locking in properly and would come loose with a bigger load at times. I was able to get a new OEM replacement lint screen and cover without problems just 2 weeks ago. Fit perfectly. Problem fixed.