Fish and shellfish are a big part of diets throughout the world, and it is also a potential big source of nutrition in a Paleo diet. There is no real prescription on the types of fish to eat in the diet in general. As with the eggs it’s preferable that in most cases you go to a wild caught source of your fish versus farm raised. There are some types of shellfish you can eat farm raised that may not have any difference in nutrition, like oysters and mussels. However fish like salmon and tilapia often have a very different nutritional profile when you buy farm raised over wild caught.
Another thing to watch out for is eating too high up the predator food chain. As carnivorous animals eat the smaller animals the amount of toxins in their fresh also increase. That’s why something like swordfish has a lot more mercury than something like sardines. Some books therefore advise trying to eat the species near the bottom of the chain and try to get people interested in eating sardines and anchovies. I’ve tried to eat these in salads and I have to say that the fishiness is an acquired taste. However if you can get past that then they are a great source of omega-3 fatty acids and nutrients.
Canned wild caught salmon is another good potential source of omega-3 rich foods. Again, many of the paleo sites try to encourage people to get the versions with the bones in the can. There are lots of nutrients in bones. However I have never been able to get over the whole texture aspect to eating the bones so I instead go for the boneless variety.
Lastly, besides buying fish for cooking at the grocery store there is always sashimi or sushi-style rolls made without rice. These are often wrapped in seaweed, which is very good for you, with some additional cucumber or other sorts of filler added to bulk it up. Just remember to avoid the soy sauce and instead use your coconut aminos instead!