There is no denying that the chickpea is the shining star of the vegetarian substitution world. It makes a great fritter, its liquid can take the place of egg whites and it makes an excellent tuna-like salad.
Photo by thebittenword.com.
Chickpeas provide the texture and a slightly meaty, savoury flavour, but to get that ocean-y hint, The Kitchn combines the processed beans with umeboshi vinegar, which has a "seafood like flavour". (If you want to further up the "chicken of the sea" factor, you can also crumble some nori in there.)
This is mixed with mayo, celery, mustard and a few other seasonings and combined in a food processor until spreadable, after which it can be enjoyed however you like to eat tuna salad. (I hear bread is the usual vessel, but I've always like mine with apple slices.)