I'm really not interested in putting insulators in these cables or trying to load them as antennas

That's a shame, because the mast and the cables probably make an excellent antenna. In fact unless you go through some trouble to get the antenna far away from the mast, or oriented orthogonally, the mast and cables are going to end up being a significant part of the antenna whether you like it or not. Remember you don't need a DC connection: mutual inductance and capacitance will couple things together at RF.

You don't need to put insulators on the cables. Finding a good feed arrangement will take some experimentation, but there are plenty of ways to couple into the mast which don't involve any structural modifications, which I guess is your fear.

If the mast, cables, and hull make a closed loop anywhere, you can put a smaller loop within that and feed the small loop. The two loops effectively make a transformer.

Another method is to clamp a ferrite around one of the cables or the mast, and also run several turns of wire through the ferrite, and feed that. Again, a transformer feed. This works even if

A gamma match is another option.

If you run a wire up the top of the mast like SDsolar suggests, you've effectively made a crude gamma match. The mast and the wire together make a transmission line. By varying the length of that wire you change the length of that transmission line. You can also decide to connect the wire at the top to the mast or not, which will give you an open or shorted transmission line stub which you can use to make a stub match.

These are just a few options. For further ideas you can research existing methods of coupling into loop antennas or shunt feeding towers, which are similar problems.

I'm really not interested in putting insulators in these cables or trying to load them as antennas

That's a shame, because the mast and the cables probably make an excellent antenna. In fact unless you go through some trouble to get the antenna far away from the mast, or oriented orthogonally, the mast and cables are going to end up being a significant part of the antenna whether you like it or not. Remember you don't need a DC connection: mutual inductance and capacitance will couple things together at RF.

You don't need to put insulators on the cables. Finding a good feed arrangement will take some experimentation, but there are plenty of ways to couple into the mast which don't involve any structural modifications, which I guess is your fear.

If the mast, cables, and hull make a closed loop anywhere, you can put a smaller loop within that and feed the small loop. The two loops effectively make a transformer.

Another method is to clamp a ferrite around one of the cables or the mast, and also run several turns of wire through the ferrite, and feed that. Again, a transformer feed. This works even if there's not a loop to feed. That is, you could clamp such a ferrite around a metal pole stuck in the ground (or the water) and make a fine antenna. A mast on a sailboat is not far from this. Might get tricky to find an appropriate ferrite at higher frequencies or higher power levels though, but might be great for QRP operation on 40 meters.

A gamma match is another option which not only solves the problem of coupling into the mast without structural modifications, but also gives a couple variables to provide a decent match.

If you run a wire up the top of the mast like SDsolar suggests, you've effectively made a crude gamma match. The mast and the wire together make a transmission line. By varying the length of that wire you change the length of that transmission line. You can also decide to connect the wire at the top to the mast or not, which will give you an open or shorted transmission line stub which functions as a kind of stub match.

These are just a few options. For further ideas you can research existing methods of coupling into loop antennas or shunt feeding towers, which are similar problems.