Splitting 220v for washer dryer install

You will not be able to safely apply the correct breaker protection. Run another 115Vac line and remove all safety and current protection concerns.
Expanding further, 230Vac lines usually do not include a white wire (neutral), only red & black with fault current protection green. Do not try to use the green wire for neutral.

Step down with a false neutral tap? Errreie.
Run a new wire.
One other desperate idea; Is there a nearby 115Vac outlet to the head or stateroom? Heavy enough to run the washer you could tap on to? Branch off of it. This would not be recommended (or ABYC) but a desperate DIY idea.
Naww, Run a new wire.

If its only in use 10 times a year can you run an extension cord from an outlet that is man enough to run the thing. Or if and only iff the cable now supplying 220 is of adequate size to carry the current your 115v machine will draw you can change the other end of it and supply it with 115v, you will need to change any inline protection to suit the application and if you don't have the correct colours as long as you label the wires at each end and know what it is your dealing with should have no problems.

recommend home run with separate breakers.

Can he do a subpanel and still be ABYC complainant?

A sub panel off of what? No white wire where the dryer is now. No response if another outlet with heavy enough wires is available close by. Wire size vs max current draw (everything on) would allow a sub panel or branch/extension. ABYC does NOT take into consideration "used 10 times a year" or promise not to use that outlet while washer is in use.

Maybe an interesting course to see if the washer is available in 230Vac or see what is involved to make it 230Vac.

sub panel off the existing 220v that is there now as described in the OP

No white wire in the existing 220v that is there now as described in the OP.

He wants both a 220v outlet and a 110 volt outlet. I believe the dryer may be 220volt, while the washer is 110 volt from the OP's origional posting. That's going to require running another set of wires. Or possibly just piggybacking off of the closest 110 volt circuit nearby if the wire size and capacity is enough.

understood.

Good idea. Before I read this post I was going to suggest it. Boat repairs are so costly that most boaters look to DIY on most things, but there's a reason that licensed electricians are required for jobs like this in commercial and residential structures. Boats aren't as regulated, but screw this job up and you burn.

A query, if the existing 220 v circuit was put into a sub panel, one wire taken off to feed as 115 outlet and a neutral taken from the centre pin of the iso tsfrmr, would that be legal ?

You re talking about only 110v hitting the dryer, are you saying both the new washer and dryer are 110?

If so, you could probably use one of the existing hot for neutral by disconnecting the wires at the panel, connecting one hot to a 110 breaker, and using white electrical tape to cover the ends of the other wire and connecting it to the neutral bus

As long as the ends of the wire are taped over, you re good to go and I m pretty sure it s an acceptable way to comply with standards

What wire is in place now? Gauge, conductors, ground? Might be undersized. Am I correct that one unit is 120V and the other is 240V? Current US code would have required 3 conductor plus ground to the 240V equipment. If that wire is in place a sub panel would be perfectly acceptable. it probably isn't though.

Current code for a 230Vac lead is Red, Black and Green. Even if there is a white run in the bundle, how would you breaker protect it? Make a split breaker, assuming one lead (black) is large enough to handle the extra current?
KIS&S. Keep it simple and SAFE.
I like vertical in the morning. Not a BBQ.

If you have two hot legs with your 220V ( or whatever the correct voltage is) you can protect these with a double pole breaker.

Try working on something where the conductors are all the same colour - Black.

In new construction we are required to have a 3 wire with separate ground for 240V circuits now. Existing it is ok to bond the white and ground at the dryer.

Landside and marine electrical are two different worlds. They just share some similar colors of wire.