Ahem. Has anyone actually measured the thickness of the "raft" In looking at the diagrams and plans, it appears that, in 1/144 scale, i would be a tad over 1/4" of slab side above the waterline and significantly more (3-4 times that) below. Double that amount for 1/72. With a max 3/8" for deck and caprail, you would still have what appears to be close to 1/2" of penetrable area below the waterline, even in 1/144.
A tight shot - you betcha. Impossible for an Iowa? You haven't met Don.
A well built and captained Bismark should also make mincemeat out of it - a ship that small cannot take many hits, and virtually every hole is effectively a below. Even a Fletcher, if you got one built in 1/144 could put out a lot more rounds per minute on the Monitor than the monitor could return. Doesn't have to be as many shots on target if you have a lot more shots.
For what it is worth, the little 22" tramp steamer USS Lake Shore has very little penetrable area above the waterline, but she still gets sunk. At least in NTXBG battles.
Now, as for which ship I would least want in a last man standing between the ones given is probably the Fletcher. But that is just off the top of my head without reflection.
One other minor point - you specified infinite ammo and 3 unit opump. In Big Gun, units are not generally used, for pumps or otherwise. Instead, either pump outlet size or GPH is used. And, it is a tad unrealistic to specify infinite ammo when two of the ships listed are pretty limited in size and thus ammo carrying capacity. Last, methinks you are making some wrong assumptions on rate of fire. As I recall, the Monitor had 11" guns.
In 1/144 Big Gun clubs that translates to, in this case, 2 rounds per 6 seconds or the equivalent throw of 1 round every 2 seconds.. The Iowa would be firing 9 1/4" rounds every 8 seconds, or the equivalent throw of less than one second per round. The Fletchers would have a faster ROF, I am thinking, offhand, but am not sure of the 1/72 or 1/96 ROF rules.
Cheers