You do have a problem that really,IMHO, has no good answer.  Weldng rusted metal is, at best, only 'so so'.  The ideal is , of curse, to remove all the rust(iron oxide just doesn't melt well).  Obvioulsy with a ships hull as  old as yours that becomes a problem.  As to welding rod/wire to do the job, it would appear you have tried the old standbye solutions.  I have a great deal of practice at welding rusted metal with mostly poor results.
A couple of things that I have used with fair results is to chemically neautralize the rust before welding (not effective in all cases) and bead blasting (if material thickness permits). Using E-6013 AC mild steel electrodes with a 'buzz box' seems easier than DC in dealing with some corroded metal.
Sorry to say, reading this back, I have no ideal solution.
As a general rule, in automotive type repair and in structural buildings rust removal  precedes any welding. In your case that may mean large panel replacements. High dollar.  Good luck.

Rod

Is the metal also damp?  It may be beneficial to preheat the metal with a torch until it is warm - you may see water literally run off.  Also, if it is cold, it may be pulling the heat out faster than you can add it, leading to your poor flow.  I defer to Rod's experience in welding; just a couple of questions to maybe create some thoughts.

Blacksmith

A good weld on bad metal is impossible, but a little more information - like the type and duty cycle of the machine you are using, the sizes of the electrodes and the heat or amps you're using for them, the length of your lead, the thickness of the wrought iron, the fit of your joints, the welding positions, types of welds, etc - would help with a better answer, but I can offer a few tips.

Make sure that your electrodes are kept dry at all times, and warm them - a hot box is good - before use. Make sure your polarity is right. 6010 is DC. 6011 can be used on AC and while it can be used on DC machines with reverse polarity it doesn't work as well as 6010. 7018 is AC or DC reverse.

Make sure you're using the right size or diameter electrode for the thickness of the iron, and the right heat. Start in the lower accepted ranges for the size rod and try shortening up your arc length, increasing your hand speed, and just barely staying ahead of the pool. Watch the angle on verticles, stay 15-30 degrees on down verticals and 10-15 degrees on up verticals; and try circular weaves rather than triangular weaves, and vice-a-versa.

Don't run past the cycle of your machine, let it cool. Make sure your lead isn't too long, the ground is good, and with DC you should be looking for arc blow - especially with wrought iron. It has the fewest elements added to it and it's molecules tend to be quite long, so while it's usually very easy to weld, it also is very easily magnetized by welding and arc blow often occurs.

If your hull can't take bead blasting to clean white, I'd be a little nervous about dumping it back into the water.  The hull has to have enough strength (i.e. thickness) to take minor bumping without catastrophic failure.  Boats are a little like airplanes in that regard; you can't just pull off the freeway to fix a flat.

If it can take bead blasting, by all means either grind or blast until you've got good solid iron to work with.  You may have to locate some appropriate wrought iron filler and gas weld.  A low-hydrogen arc rod isn't going to be the way to go unless you really prepare the bevel and fit well and have a good root with a backer. I'd still prefer a 60 series.  I can't think off hand what would be the best for cast iron, but at any rate, you may have to simply cut out rotted sections and replace with plate using an appropriate rod.  That could be tough if your hull has a lot of shape, but regardless, completely removing the rust will go a long way in improving the appearance and ensuring weld strength.

Frankly, I'm a little surprised your plates aren't simply riveted at that age.

I have had the best luck with 1/8 in. 5P rod.  Run about 110-115 DC Amps and use a whipping motion.  You will see the puddle flow together with the base metal very well.  It is very hard if not impossible to produce a good looking weld with 5p, but it makes a high strength weld on rusty metal.