L.L.
My daughter is 8 months old and I give her very small amounts of tuna already...she loves it, and it is perfectly safe. Don't overdo the large fish (swordfish, tuna, salmon...) but it's hard to "overdo" fish with a young child. Stay away from albacore, and just use the chunk light tuna in water, and make your tuna salad or whatever and spread a small amount on toast...my daughter loves it!
Absolutely safe, and healthy.
You can make salmon "burgers"...CAREFULLY debone and take the skin off (not necessary, but it's more appealing without the skin), mash together with an egg, saltine crackers, and enough milk to make patties, and cook in a skillet with a little olive oil until lightly browned on both sides. It will flake easily, is delicious...and is even better served with a homemade cheesy white gravy. You can find that recipe on a box/package of cornstarch.
Personally, I wouldn't do shellfish...but maybe that's just a preference. I know my oldest was eating shrimp by then.