My first thought is that making a vegan dashi should be easy. Put some dried kombu and shitake in water in the fridge overnight, then boil the mixture, strain the solids and skim any foam/particulates off the top and now you've got an seafood broth you can use for soups/pastas/etc

Vegetarian sushi comes to mind. Sometimes I even prefer it to sushi with fish when I'm in the mood; it's crisp and refreshing when done right. If you use good quality nori and rice vinegar you'll get that oceanic flavor you're looking for without any fish.

Seaweed is the way to go. As others have noted, miso soup with kombucha is good, seaweed salad, maybe tofu wrapped in seaweed, nori snacks (I like the wasabi ones).

Think of the tentacles shape, too. Maybe make a dense shallow cake and cut it into tentacles shape before frosting it? I don't think it all has to taste of the sea to be thematic.

Kelp noodles! Kelp salsa!

My first thought was using seitan as a shrimp substitute.

I found this site that has it and a few other ideas for vegan themed seafood.

Go for some vegetarian recipes from Goa. Kokum sharbat and Kande Pohe might be a good fit.

https://themadscientistskitchen.com/category/goan-vegetarian-and-vegan-recipes/

For anyone curious: after a lot of research and then failing to find hearts of palm in any supermarkets, I ended up going for vegan crab cakes with jackfruit. At the end of the day, they didn't taste a huge amount like crab, but with lemon juice, dill, and nori, they evoked vague impressions of seafood, I think. And everyone had seconds or thirds, so they seemed to taste good enough, even for the non-vegetarians :).

I used essentially this recipe, but with jackfruit instead of hearts of palm, and with far more nori: https://www.veganfoodandliving.com/vegan-recipes/vegan-crab-cakes/

Thank you for the ideas!