I got Island Living yesterday, and I'm definitely seeing this with the Kealoha household that I'm currently playing. (*POINTS TO SCREENSHOT*)
I think those really dark spots are puddles of shallow water from the tide? (As I was typing this, though, I wonder if they're actually shadows, though...)
(I haven't played any other pre-made households, I just went into them long enough for the game to tag them as "Played.")
UPDATE: @PoptartJuniper pointed out on Discord that there's a sliver of the deck on that house that touches the water, and that triggers the swimwear change. I discovered that it's actually the stairs leading up to the house deck? I decided to try building a different set of stairs on the opposite side, and had the family manually go up them. No swimwear! (I attached a pic; the offending stairs that come with the house are on the opposite side.) I'll have to experiment some more with that, but hopefully moving the stairs helps alleviate the issue in that lot, at least.
Just as @StarryMight posted, the problem with the Kealoha family is the stairs leading to the house. The sims always changed into swimwear when arriving or leaving the lot before I deleted those stairs and put new ones on the other side of the house.
I'm grateful for finding this post so I could fix the issue because it prevented the conservationist from changing into conservationist clothing. Thanks for posting!
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