You could do something like smoked mackerel or trout dip? Super easy and really tasty :)

Ceviche is always tasty

make a salmon mousse and pipe it onto hothouse cucumber slices. garnish with something pretty. it's deelish, beautiful and low carb too :-)

Second the ceviche idea.

I had grilled shark that I loved at a birthday party - seasoned simply with sea salt and pepper, grilled in large steaks, additional salt sprinkled and lime juice squeezed on top and then cut into bite size chunks and speared with toothpicks. It was delicious!

I've never tried it, but I've always thought brandade sounded tasty: Brandade

Just to clarify, you mean no shellfish, right? You can probably look up recipes for crab appetizers and substitute a mild white fish. It might not have the same wow taste that crab does, but I can think of cheesy crab-on-toast appetizers I've tried that would still taste great with fish.

Ceviche! There was a wonderful looking recipe in the NYTimes Recipes for Health section yesterday

Baccala (salt cod) is an Italian Christmas tradition. I know Mario Batali has a couple easy and delicious recipes, as does Lidia Bastianich, if you google for them.

Kippered (smoked northwest style) albacore - line-caught, and delicious with a selection of good mustards. It's not as greasy or salty as traditionally smoked fish. The best source I know is Katy's Smoke House in northern California. http://www.katyssmokehouse.com/

As RMF mentioned, Baccala is a good choice but it's not really appetizer.

I'm a big fan of high quality Italian tuna in extra virgin olive oil. Toast some good bread and add a bit of tuna. Drizzle with a bit of oil, fresh pepper, and dried oregano. Top with a sun dried tomato. There are plenty of high quality canned tunas on the market.

Whatever that is in the picture looks wonderful!

I am not too creative here, but how about whitefish salad between two rounds of marble rye to make savory, bite-size sandwiches? Very easy to eat.

You could also do a small mousse molded in an interesting shape with homemade crackers.

Miniature fish sticks breaded and fried with a dipping sauce?

Thai fish cakes. Ceviche was also a good suggestion.

What about caviar or a roe dip, something like taramosalata?

Seared rare tuna is definitely a treat, or you could do a sashimi application (if your guests are a little adventurous).

Ceviche is a good idea, plus you can put it into individual edible cups (tuna and avocado ceviche could be very good in a tortilla cup!)

Spanish bonito tuna, Ortiz brand is good, on crostini spread with a roasted red pepper puree/romesco and aioli - like this:
http://frshforce.files.wordpress.com/2011/11/20111121-041239.jpg

or just the bonito on baguette with roasted red pepper slices and some drizzle of nice olive oil (ignore the sherried mushrooms next to the pic):
http://i1194.photobucket.com/albums/aa372/cherrybomb99x/16643_224660920648_736640648_4034034_956916_n.jpg

I have had some awesome salt cod fritters at a tapas restaurant. Maybe you could check out some tapas cookbooks for ideas.

not sure if you are looking for non-finger food apps, but you could do little shots of bouillabaisse or take inspiration from turkish fish meze? they have bazillions of little fish dishes (tiny smelt, sardines, etc) -- but may be a little too intense (for my fam it would be perfect) ...

Instead of the Italian route, try the Brazilian version of salt cod: Bolinhos de Bacalhau

http://www.salon.com/2010/01/09/portuguese_cod_cakes_recipe/singleton/

Brandade (as suggested above) is delicious! Highly recommend that for an appetizer.

Thanks a lot to all of you for your awesome suggestions! I'm pretty sure I'll find something now ... it might just be difficult to choose now!