I'm inclined to agree with sumpter250. A landing craft assigned to an APA probably would spend only a small percentage of its time in the water. It's also worth remembering that location of an LCM's waterline varied quite a bit, depending on whether a tank was on board or not.
On the other hand, the water near the beach at a Pacific landing must have been filthy, with oil, gasoline, assorted junk, and even blood floating on the surface. (Hmm...come to think of it, does human blood float in saltwater? Maybe not. Let's drop that one.) It would be interesting to look at some photos of landing craft that had just been hoisted out of the water after a landing. I rather suspect some crud was visible on their bottoms, but the maintenance crews probably hosed it off as soon as they had time.
My father was a junior boat group officer on board the attack transport
Bollinger (APA-234), which never saw combat but made the rounds among many of the Pacific battlegrounds shortly after the fighting was over. (If she'd been on hand a bit earlier I might not be here.) I'll always regret that I didn't pick Dad's brain more than I did, but he did pass on some tidbits that might be relevant here. One of them was a bit of info about weathering that I've mentioned two or three times in other threads of this Forum. I'll repeat it here for the benefit of anybody who hasn't already read it; I've never heard about it elsewhere. Those who've read my other posts about it - stop reading now.
Much of the time of enlisted men on board WWII U.S. Navy warships (as is the case now) was taken up with chipping and replacing paint. ("If it moves, salute it; if it doesn't move, chip the paint off it.") Virtually every metal part of an attack transport, including the landing craft, got repainted on a fairly regular basis. The standard procedure was for a gang of sailors to chip and scrape the paint off an area of several square feet (making it impossible for anybody within a hundred-foot radius to sleep in the process), brush on a coat of primer, let it dry, and then apply at least one coat of the finish color. The only part of the ship not subject to this ritual was the exterior of the hull, which was off-limits while the ship was under way.
The standard metal primer used by the USN during the war was a sickly, slightly greenish, pale yellow - presumably based on zinc chromate. (Dad described how "some of our great geniuses" tried to save time by mixing a can of primer with a can of grey, thereby producing a particularly nauseus green that The Chief made them throw over the side. There were no anti-polution rules in those days.) According to Dad, at any given moment a dozen or so irregular spots of yellow, in whatever size the painting gang could handle, would be visible on the ship - and their location would change at least once a day. I assume the landing craft were given the same treatment, especially just before and after an operation. (I imagine the plywood parts were dealt with in a similar manner, but I'm not sure about the color of the wood primer.)
The key to reproducing this effect on a model is to keep it subtle. The yellow has to be extremely dull, so it's only noticeable on close inspection.
I once built a model of the
Bollinger, using the old Revell APA kit as a basis, and gave it to Dad for Christmas. He was in his seventies at the time. The look on his face when he put on his bifocals and saw those yellow spots was one I'll never forget.
Dad died in 1990; I don't know whatever happend to the model of the
Bollinger. High on my list of future models to build, though, is the new Italeri LCVP. It's going to have the letters PA-234 painted on each side of the bow, and two buckets of paint, one yellow, one grey, sitting in a corner of the cockpit.