It is a property tax, which by definition taxes property. Boats can be quite high value so taxing them is effective. In theory property tax could apply to anything such as your TV but the values are too low for the overhead so having a short list of high value items such as houses, cars and boats makes sense. This is what most counties do.
The tax is random, it is on property which makes much more sense than on income for example.
Why does property tax make more sense, it doesn't relate to ability to pay, or the value of the asset at acquisition.
It is entirely possible for property taxes to be massively regressive, or to have massive unintended consequences that significantly damage the revenue generation.