Item-wise, there is a tag for #minecraft:chest_boats and a tag for #minecraft:boats, which contains all boat and chest boat items.
Entity-wise, there is no #minecraft:chest_boats tag at all, and the #minecraft:boats, tag will not select chest boats.
Seems like a minor oversight from the switch away from NBT, it feels kind of crazy that you have to manually clear each type of chest boat in a scenario where something like /kill @e[type=#minecraft:chest_boats] would make perfect sense!
Thank you for your report!
After consideration, the issue is being closed as Invalid.
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