Duty is a fact of life. Check with HMRC to see what it would be. In the US, there is zero duty on water sports equipment. Although just what constitutes “water sports equipment” can be a bit open to interpretation. I’m a licensed customs broker in the US, so I deal with this stuff all day along.
It is 2,7% when importing water sports equipment to EU form US. High European VAT on top of that (and shipping too) is the problem.
That said, most brand name fins made in Taiwan cost about the same in EU and US when bought locally, after duties and taxes.