If the boat you have bought is used, you do not need to pay Swedish VAT. If the boat is new according the definitions of the Swedish Tax Agency, you must register your purchase and pay Swedish VAT.
If you are importing a boat from a country outside the EU or the EEA, you are also responsible for ensuring that the boat is CE-marked. You can find more information from the Swedish Transport Agency.
More information
Swedish Tax Agency
Swedish Transport Agency
