Alright, I've made some saves and I'll experiment with the settings some more in the meanwhile. From what I've seen so far, the combination of waves and fighting multiple ships causes the frame drop, and occurs in all the mods/derivatives that I've tried so far (Lost Ships City, Caribe Stories, and Eras 2.) I'll make sure to save whenever I encounter the issue and I'll tell you if I can find any combination of settings that gets rid of the frame drop.
UPDATE: When near land, foliage quality drastically affects the framerate. Disabling foliage results in a big FPS boost. I don't know if this is something exclusive to Windows 10, though.
Considering the effects of foliage, it's probably a safe bet to say that the rendering of crewmen running around on ships is the cause of the frame drop when near several other ships. An option to disable them, if one does not already exist, would probably help. I'm not sure why this problem exists on Maelstrom and not the original Storm Engine version of COAS, since Maelstrom is more stable otherwise in my experience thus far. I watched a playthrough of Eras II and the uploader didn't seem to experience any frame drops, though I don't know what OS he was using.