We won't be selling our shellfish to the US
We already sell shellfish to China, which is further. I guess they either freeze it, or ship it live. And we sell a lot of salmon to USA. They find ways. It's profitable for us, otherwise we wouldn't do it. £200m in salmon to USA alone. Of course, if Scotland leaves the UK, that's mostly their piece of the pie, not ours, but total salmon exports are under a billion, and it's not all Scottish. I digress. We can ship fresh food to USA, China, anywhere on Earth, as evidenced by the fact we already manage to do this, and evidenced by the fact we import fresh food from South America. These markets aren't new to us.

2. A free trade arrangement with the US is politically unlikely to happen any time soon anyways.
Perhaps, but USA was just one example. China, India, Brazil, these are also huge markets we could potentially tap into.

I mean, none of this really matters. Economic dependency is no reason to cede sovereignty. It's like you're happy to be economically trapped in a superstate. I'm not.