I found this marinade on cooks.com. I tried it tonight on swordfish steaks and think they were the best I ever had. Will probably work as well on salmon or tuna.
1/4 cup olive oil
2 tsp. basil
3 tbsp. white vinegar
2 tsp. oregano
1 tsp. garlic minced
2 tsp. equal parts salt, pepper & thyme.
Combine the above ingredients and marinate your fish steaks in them for an hour. Grill to your liking.
I hope this isn't negative!