Author straight up wrote a potential plot hole in trying to create suspense, if there is any threat to the boats from that unique skill then the MCs skill is flat out wrong. It's not listed as "immune to monster attacks" it's listed as "indestructible" and "unsinkable" so even being generous if the thing could damage the boats they still would function as boats. If something is unsinkable even if you opened a giant hole straight in the middle of the object it would continue to float, otherwise it isn't "unsinkable" its just really durable. Obviously the women in that party don't know the details of the MCs ability, but the readers do so why is the author trying to set up suspense like there's a genuine threat to the ships being damaged when they explicitly mentioned that the ships can't be damaged? Even in universe the god that made the skill says something along the lines of "the ships can withstand an archdragon attacking" so unless unique skills are THAT broken comparatively the logic doesn't make sense unless this is just to make the women seem stupid for being scared. Especially when the entire premise of why the MC has this skill is because that god was trying to make a point to other gods that "not all skills have to be ridiculously OP to leave an impact" so if his boats can be destroyed by this it kinda just proves the other gods right that summoning a boat is a stupid skill in a world like that when compared to the skills they've been giving out